Play on idiot-savant.
Depending of course what your definition of the word 'is' is
President Bill Clinton said something close to this while trying to explain that he had not lied when he denied having sexual relations with Monica Lewinsky: "It depends upon what the meaning of the word 'is' is..."
You can hear it here.
the kid in the teen horror movie who turns out to be possessed
A likely candidate for which teen horror film Pynchon is referencing here is Night of the Demons (1988) (aka Halloween Party). Wikipedia
air in a can
CFC's (chlorofluorocarbons) were banned in Europe in 1990 and in the U.S. in 1994, with a phase-out period. Fluoroethane (R-134a, no chlorine) was one of the replacements and can be purchased compressed in a small canister (with a nozzle) for removal of dust from camera lenses and other optical equipment. Since it's a refrigerant, fluoroethane cools down to something like -50 C when it expands, so even if it had psychoactive properties (which afaik it doesn't) it wouldn't be much good for spraying up your nose. Most photographers use compressed air instead (for lens cleaning, not as a recreational pharmaceutical). Maybe a play on "ice", who appears again in the next paragraph.
patafamiliarass around here
A pun on pater familias, defined below. Avi becoming a “patafamiliarass around here” is Pynchon punning on it with “pat a familiar ass” which would be Avi’s being treated in condescending or patronizing way in a place where he's taken for granted. A "pet" of sorts, not totally unlike Chandler Platt's assistant Darren, whose rap on page 282-283 is so defiant while Darren himself remains, essentially, Platt's subservient pet.
The pater familias, also written as paterfamilias (plural patres familias), was the head of a family in ancient Rome. The paterfamilias was the oldest living male in a household, and had complete control of all family members until he died. Once the paterfamilias died the next oldest male would then have control. The pater familias was always a Roman citizen. From WIKI.
1940s-era slang for "hands" often seen in the pulp/hardboiled/noir/detective fiction of the time.
To understand the origins of this slang term, hold an imaginary sandwich, with both hands, in front of you. A couple of hooks, eh?
the company tambourine
The company's moneymaker. "Shake your moneymaker" is an old blues lyric, recycled, just like blues riffs, throughout the history of the blues and rock 'n' roll. Variations include "shake your tailfeather" and "shake your tambourine." 
Similarly, the rapper Eve in her 2007 song "Tambourine" used both "shake your tambourine" (shake your ass) and "shake your tambourines" (shake your tits). 
RPGs are Role-Playing Games
Mrs. Cheung's bleak announcement
cf. p. 335: "Ms. Cheung, an English teacher who if Kugelblitz were a town would be the neighborhood scold, has announced that there shall be no more fictional reading assignments."
Edwin John "Eddie" Fisher (August 10, 1928 – September 22, 2010) was an American entertainer. He was the most successful pop singles artist of the first half of the 1950s, selling millions of records and hosting his own TV show. Fisher left his first wife, actress Debbie Reynolds, to marry Reynolds's best friend, actress Elizabeth Taylor, when Taylor's husband, film producer Mike Todd, died. This event garnered scandalous and unwelcome publicity for Fisher. He later married Connie Stevens. Fisher is the father of actresses Carrie Fisher (with Reynolds), Joely Fisher (with Stevens), and Tricia Leigh Fisher (with Stevens). From WIKI.
Goombas, known in Japan as Kuribo ("Chestnut People"), are a fictional species of sentient mushrooms from Nintendo's Mario franchise. Their appearance is based on shiitake mushrooms. Wikipedia
here comes a plastic top from a nine-inch aluminum take-out container...
When Maxine sees the plastic top from a take-out container rolling on its edge down Broadway, seemingly with a mind of its own, reader Diana Poskrop recalled that in Gabriel Garcia Marquez's 1970 novel One Hundred Years of Solitude, after Jose Arcadio's murder his blood took a direct route through town to Ursula, his mother. (HarperPerennial Classics, 2006, pp. 131-132).
meet my man Ketone
In chemistry, a ketone is an organic compound with the structure RC(=O)R', where R and R' can be a variety of carbon-containing substituents. Ketones feature a carbonyl group (C=O) bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology. Examples include many sugars (ketoses) and the industrial solvent acetone. From WIKI.
ketones, when inhaled, also produce a sort of high, as discovered by glue-sniffers. The disadvantage is that inhalation of ketones (including acetone) causes significant, permanent and irreversible brain damage.
strange feeling about the Internet, that it's over
Sounds like a replay of Doc Sportello's thoughts about the Sixties at the end of Inherent Vice?
Wagnerian brass section
Wagner's writing for the orchestra's horn section has always been sublime, and (at least in my short and long term memory) never "blaring". Perhaps Maxine is thinking of the helicopter scene in Apocalypse Now, where Ride of the Valkyries does blare from the mobile speakers.
One of Wagner's contributions to music was the invention of the "Wagner tuba", an instrument about halfway between a tuba and a French horn. It was derived from an instrument popular in German military bands of the day, and as a result his music probably would have had a distinct martial feel to operagoers of the time, but I don't think that's what TP is getting at here.
Granada Asbury Park Uncertainty Question
From the lyrics of "At Long Last Love" (written by Cole Porter for the 1938 musical *You Never Know*; first sung by Clifton Webb; recorded by Frank Sinatra for the 1957 album *A Swingin' Affair!* and the 1962 album *Sinatra and Swingin' Brass*)
Is it for all time or simply a lark?
Is it Granada I see or only Asbury Park?
From this page: a term used to describe a scented trail left by the fragrance wearer.
The Ray Milland Story
Ray Milland (3 January 1907 – 10 March 1986) was a Welsh actor and director. His screen career ran from 1929 to 1985, and he is best remembered for his Academy Award–winning portrayal of an alcoholic writer in The Lost Weekend (1945), a sophisticated leading man opposite a corrupt John Wayne in Reap the Wild Wind (1942), the murder-plotting husband in Dial M for Murder (1954), and as Oliver Barrett III in Love Story (1970). Milland, who was at one time Paramount Pictures highest paid actor, co-starred alongside many of the most popular actresses of the time including Gene Tierney, Grace Kelly, Lana Turner, Marlene Dietrich, Ginger Rogers, Jane Wyman, Loretta Young and Veronica Lake. From WIKI.
breaker breaker good buddy
"breaker breaker" is trucker CB-radio talk for a request to interrupt the conversations on a channel and start a new one with anyone on the channel, in this case Maxine. "Good buddy" is also common CB-radio vernacular used by truckers in addressing each other.
a.k.a., Microsoft headquarters
racks of electronic gear receding into infinity
Describing the Bleeding Edge front and back cover photograph. On the next page, Eric speaks of "Bleeding-edge developments"...
and another tardis-like "bigger-inside-than-outside" shot
going as you might expect for rock bottom prices
After the dotcom bust of 2000, all of the hardware bought by now-bankrupt tech startups was released onto the used equipment market, and this did cause a big drop in the going price for both new and used servers.
Bleeding-edge development phase
On the previous page, Pynchon describes a server farm that matches the Bleeding Edge cover photo.
not even Fort Sumter
The first battle of the American Civil War. Wikipedia
We could be heading anywhere, Alberta, Northwest Territories, Alaska
If Horst is correct, they are westbound on I-90 in southern Montana. At Butte, they could head north on I-15 to get to all of these places. Looking for cold weather to run their server farm?