1) both me and JazzleSAURUS already had suggested on previous page for your weather all-seasons
. Due living in place that is colder on average and often snow/ice at least half of winter i never drove with ones myself using only real winter tires (currently even studded ones), but most often in this forum from all-seasons i saw mentioned michelin pilot A/S 3. Supposedly grip very well, about only drawback, due softer rubber sidewalls more mushy feeling/less sharp steering. For stock wheels of x7" width min 195, ideal 205-215, max 225. I'd go with stock size, which as you know it, 215/45/R17.
2) if in future you want to use spacers (and thus possibly extended studs if with thicker slipon spacers), i guess worth getting open ended lugnuts.
Not sure on oem crash bolt use. OEM crash bolts are same 14mm bolts that are in lower holes (stock upper bolt - 16mm). Most often they are reused together with normal camber bolt with lope in lower hole, relocating originals to upper. But if you want to use just OE crashbolts, you'll need to order another pair for upper hole .. and by then why not normal camberbolts (which for reference are SPC 81305 for lower hole, Whiteline KCA416 for upper)? They are not expensive.
Regarding flushness, i don't care at all about it, and not too fond of spacers, but from what i know, if properly used/installed they are safe enough. Needed spacer thickness you can get from wheel offset and suggested offset from sticky wheel directory thread's first pot wheel flushness chart. If due spacer usage you'll get extended studs, check of their (or lugnuts' which you stated planning to get too) thread to match. Stock studs (and lugnuts) are M12x1.25 threaded, but often 12x1.5 are chosen as harder to strip.
3) from googled up 45F is 7C? Sounds about right. I wouldn't think/wait that precise though
. At nights it's usually cooler, and if one has to drive early in the mornings .. when day temp in autumn starts drop around/below those 45F i start to think when it's less hassle to do switch, eg. in weekend. Though some sudden first snow even if thawn off right away, or temps below 0C(32F) in night may convince me to change right away, even if day temps are warm. One may also look at weather forecast.
For storage i myself bought tire totes for simpler carrying around to-from car, and special tire storage cover/tire storage bag for all four tires/wheels stacked (i have small flat, so have to store them outside on balcony, to save space. So that cover is for protection from sunlight/rain/snow).