Here's commentary from somebody who hasn't actually read the books, so take it for whatever it's worth...
(1) How fast is it supposed to be (well, other than "speed of plot", naturally...)?
Outside the books, it seems the speed actually varies quite a bit - 15 ly/ X min and 300 ly/h in "Hope and Fear", and all the way to Alpha in a matter of hours in "Timeless". Each of these would seem to represent a "maximum speed" of some sort, in its own context.
So it's probably basically that slipstream travel allows for near-infinite speed, and your navigation skills (or your skills of juggling the "phase variance", whatever that is) dictate how much of that speed you can use.
(2) Did the Federation make advances in slipstream drive BEFORE Voyager communicated with them from the Delta Quadrant?
Since the Voyager
herself was so easily and quickly modified in the field for very high performance as seen in "Timeless", this probably wasn't necessary - any old design could have been adapted. OTOH, the ship first seen in the novel context (that is, the Merian
from the calendar) does not appear particularly large. She's markedly smaller than the Voyager
, by all appearances, so she could well have been custom-built after news of the new drive reached Earth.
Since slipstream was supposed to be a working, already fully developed technology, Starfleet could swiftly verify it aboard the Merian
and immediately start designing the bigger ships with which to wage war in the Typhon Pact books.
(3) Related to #2 somewhat, was it spoken of in Trek Lit if Voyager communicated the technical information and specifications of the quantum slipstream drive to Starfleet between the time of "Hope and Fear" (by which time they had accessed the Hirogen communication network as established in "Message In A Bottle", earlier in the season), thereby giving a few more years of lead time for the program, or if Voyager kept that data to itself until finally arriving at Earth in 2377?
That sounds quite unlikely - "Hunters" seemed to establish that the network was destroyed for good due to Janeway's meddling, and no later episode suggested that our heroes could have made further use of it. There was no data exchange in "Hope and Fear" via the network, merely the opening of a previously sent message from Earth that was delivered by the deceitful Arturis and first offered in corrupted form.
During the narrow time window when the Hirogen network was available, our heroes would not yet have heard of slipstream, then.
I like the "icebreaker" idea - the slipstream path is clearly a phenomenon that other ships can utilize, as we saw in "Timeless" where the Delta Flyer utilized a slipstream created by the Voyager
(5) Quantum slipstream appears to work most effectively when the ship using it is more slender in profile. Does this mean that the wide Galaxy- and Nebula-classes are not able to slipstream at all, or does it just require greater power to allow them to slipstream?
Hard to tell. But their forward profile isn't particularly big - they just happen to be wide disks rather than needles, but the rim of the disk is quite thin.
It might be more an issue of "tucking in" all the extremities of the ship so that they hug as close to the ship's key parts (nacelles or deflector?) as possible, giving the slipstream hole an ideal cross section. That is, not just minimal area (possible for a Galaxy
, but also a "relaxed" circular shape (possible only for needleships).