We've been aft several times. IMO the soot issue is way overemphasized. Long walk? Only you can decide that. There's much less foot traffic, so noise is not an issue.
You're way over-fixating on this. All the cabins on an angle on the hump are larger than those not on an angle. Some on the angle are incrementally, but IMO not significantly larger than the others.
The balconies on the angle humps are all the same. Except for 7215, these are all back of the hump ("rearward facing") cabins. I'd select any of the deck 8 or 9 midship choices or 7215.
Rather than rely on how "unusual" something is based on what "others" say, why not check it out for yourself? See if this answers your questions. https://www.alaskarailroad.com/ride-a-train/buy-tickets
Personally, I'd opt for the 2.5 hour bus over a 4+ hour train.