Q. Will the new LNBF fit on my old dish?
YES, they fit interchangeably.
Sounds like you do not have the DUAL stacked, just a single, so a QUAD is needed (or DUAL Stacked) to get both satellite.
Q. One of the locations I want to put a receiver has RG59, behind cedar walls. It would be impractical to open the wall; but I could probably get at it from the basement and shorten the amount of RG59 down to about 3m (i.e. the last 3m up to the jack in the wall). How much RG59 (if any) can be tolerated by an older receiver and still function?
Each line from a QUAD contains everything you need, so 1 line per tuner is required.
Since a QUAD tops out at 1450MHz, unlike the stacked that tops out at 2150MHZ, a short run of RG59 should eb OK.
I've spliced 2'" of RG6 outside into 30' of inside RG59 to a SD rx and it worked fine.
You distance shouldn't be a problem.
Q. Any benefit to buying the multiswitch now vs. later?
Possibly that your can run 4 lines from the QUAD to the multiswitch and then distribute the lines out to the receivers as needed.
If you go direct and than later add a multiswitch, you may have a lot more reconfiguring to do.
Q. Are there any hidden benefits/penalites of buying from ShawDirect direct, or from a retailer?
No personal experience there, but many of the big box stores that carry SD should be able to economically ship to you.
Try a FS or BB in Calgary maybe.
Compare that against SD.
Usually the retailer has a better price deal.