The 185s have an outer-suburban bodyshell and therefore are outer-suburban units, just as 350s are. Yes, the 185s have better seats than many supposedly IC units (the Nova fleets being the most obvious example) but seats can be changed, the bodyshell cannot.
While the 185s are clearly outer-suburban units to me, I don't really know what the 170s are - they (along with 180s, 397s, most 22x, most 80x, all 195s and all 197s) just appear to be sub-optimal for whatever use you could try putting them to. In the case of the 125mph units, it's because only busy routes are going to be upgraded/maintained for 125mph running and on a busy route you need more than 5 coaches (the 7-car and 9-car 22x/80x are potentially useful IC units, just give the 80x some decent seats and the 22x a decent interior layout) and the crumple zones and lack of gangways make multiple working sub-optimal for 125mph stock. In the case of the 19x, it's because virtually all (I didn't list the 196s because there are so few of them) units that aren't OHLE-compatible need to be withdrawn by 2050 (within the lifespan of the 19x). As for the 170s, assuming what I've read in places like this forum is correct, 170s have fairly sluggish acceleration and don't like frequent stops. That makes them sub-optimal for suburban work, but they have suburban bodyshells which make them sub-optimal for anything else.
I completely disagree regarding the increased focus on standees being understandable - a regional express route such as Cardiff-Nottingham (3hrs 18min) can be just as long, if not longer, than an IC one (eg. PAD-Swansea at 2hrs 45mins) and a very long way indeed beyond what would be an acceptable length of time to stand. Therefore, increased standing room should, on a regional express service, be a non-issue. I assume the doors on the 185s are 1300mm wide, the same as 385s and 197s and therefore (I assume) this is some sort of standard - that is also wider than the doors on 175s and therefore creates additional standing room leaving less room for table bays, legroom, toilets etc. on a 185 compared to a 175.
At least you seem to appreciate the benefit of end doors and proper vestibules, so perhaps I can steer back towards the topic of 175s by emphasising/repeating one of the point I was trying to make - that the class 175s are remarkable units since they are the only UK post-privatisation 100mph DMUs with proper vestibules and single-width doors.
I think there are more-comfortable seats out there than the ones on the 175s (certainly just prior to them going for the TfW refresh when some of the seats had worn out leaving a really uncomfortable spot - unfortuntely I'm not sure they fixed this issue at the refresh). Other than the seats though (which, as I said above, could be changed and in good condition were alot better than the Fainsa stuff) I agree with you completely 6Gman - you have summarised the suituation well.