A usual we don't comment (much...) on next years stuff until aug 15. But I can say a few things from completely general point of view...
Twinser boards don't really _need_ to be that short. In fact, most Quatro twin fins are of similar length as the EVOs (but you are right that most single fin Quatros are a tad longer). There are shorter twin fins too, but there are also shorter single fins. Kaulis boards for example, where short even before he moved to twin fins. Most twin fin boards I've seen and used have had the mast foot in a rather normal position (but again, Kauli goes a different route and has his mast quite far back on some of his boards). Footstraps seems to be a bit more spread on on many twin boards, but EVOs are already kind of wide spread.
When it comes to fins, most tend to use 15-16cm so your estimates are realistic for a production board.
Width is an interesting point. The experience I have from the boards I've used is that you don't _need_ to go up in width and if you're gonna sail high wind and chop you might not want to. But when it comes to bottom turning on a wave at speed, I would say you _can_ handle a bit more width with a twin fin setup.