i agree with your mechanics assesment on the shock mounting.
thing of it is. when you put enough air pressure in the bags to compensate for potholes and such. when you hit a frost heave the seat will drive your spine straight up threw your neck. not enough pressure and you hit a dip it feels like the whole backend is going to be rippped off. there is no happy medium.
my original airbags got weather cracked and started leaking. contacted motortrike and found that the new airbags, the ends are plastic. so i found a set online from a company in florida that i bought a set from. now these airbags are twice the size as the ones motortrike uses. the original airbags i had to keep at least 35 to 40 psi in them to at least get a half desent ride.
The new airbags, i keep 15 lb. in the for just me and 20 lb. in them when the wife is on. first time the wife rode with these new airbags i went on a road that we were on before that is extremely rough. before she would be all tensed up waiting for the potholes and heaves. after we got threw the rough part she asked me when they fixed the road.. i told her they did'nt. thats the difference between the old and new airbags.
One thing i did do was drop a size in tire width. because the new bags are only 1/2 inch away from the tires. You would not believe the ride difference. like going from a pinto to a lincoln.
here is a link to the bags I bought. you can use the fittings out of the old bags or buy new. same with the airline. you should be able to replace both bags in an hour.
I bolted the top in place then pulled the bottom of the bag down and bolted the bottom in place.
800lb Sleeve Air Spring - 1/8" Port Air Bag
now they claim this airbag is 6 inches collapsed and 10 inches extended.
when i put mine on i only had to pull the airbag down 1 inch to get the bottom bolt in. and theres still sleeve up in the bag. so i'm thinking more like 13 extended. but these new bags are awesome.