Sorry you can't find the 9.5K Houghton in stock. I guess we got lucky. I just finished installing ours and so far it seems pretty good. Much quieter than the Dometic which, from the videos I've seen, sounds like a jet engine. Still, it's an air conditioner that is right above your head, so there is some noise.
There is plenty of space between this unit and the rear solar panel so I doubt you will have a problem with 3-1/2" of additional length. Also, the 9.5K sits on several foam pads aft of the roof opening. So even if the larger unit extends a bit over the lower area of the roof it should be adequately supported. I'll try to post a few photos of our install later.
2021 21C "Travels with Karma"
GMC Canyon Denali Turbodiesel