hardware transcoding depends on the intel i915 driver andthat one is not present in 3615/17
yes, but you should ignore the higher number of 918+ loader, there are 3 loades for dsm 6.2 and for for every type of the three it should be the latest loader
https://xpenology.com/forum/topic/13333-tutorialreference-6x-loaders-and-platforms/
unknown, atm there are no sign that we will get a loader for 7.0
it depends on the person/group who creates the loader, if someone asks, i do have a opinion about it, but it also can be just driven by the need of the one person or to have low effort/time and building on the knowledge already present and just extending/modding the loader we already have
in general units get 5 year support from synology so the 3615 were the 15 is the release year 2015 might not be continued, also its kind of redundant as 3617 has all and even more features then 3615
beside this you can still "migrate" between models and take data/settings and most plugins with it, so if you start with 3617 and feel 918+ suits better you just exchange the loader and install same or newer build of the other dsm type (matching the loader you used)
same is to expect when updating to 7.0, it needs a new loader and if its not the same type it will be the special update called "migration"