Regarding (1), I don't think Scott is very far off the mark in general. It provides prices mostly for VF and sound stamps, which are pretty close to
retail in my experience (+/- 25% or so) from dealers (show and brick/mortar shops). For the internet venues, the prices tend to be much lower in general for VF represented stamps, but I think this has to do more with the inability to hold/touch/checkout prior to buying and the unseen dealer trust risk.
I also see some very high prices for much better than VF material on the internet vs. Scott pricing, and also strong prices if the stamps have certificates. So, I am not sure significant Scott changes are needed, but they SHOULD be continually checking/updating as applicable from dealer and brick/mortar auction pricing.
I believe it is true that many dealers buy/sell as % of Scott generally, so not having Scott values as a reference would make their job much harder. I DON'T think Scott should include
ebay, BidStart and other internet venues for pricing reference. In my opinion, the dealers and brick/mortar auction houses are a more consistent and studied resource, and are a better weathervane for individual stamp market strength/trends.
(2) I would use Scott still(!) for a comparative reference, SG, Brookman, and others out there. In addition, Cole, Towle, etc. for the other usages/markings.