welcome to DP
sorry about your situation
short answer is yes, you can take the ecm removed from truck and a competent tuner can hook up and see if it is functioning.
I have a buddy that test them for a local salvage yard so they can sell for top dollar and they pay him in ecms….
I would use a different shop to get my second opinion
2007.5 3500HD Silverado LTZ
Kennedy Custom Tune
Banks Intake and 4 " exhaust, Air Dog 150, Leveled front end, aftermarket upper control arms, tie rod end sleeves, bypass oil filter, Pusher 3" "hot side" charger tube, aFe Turbo inlet pipe, PPE deep Trans Pan with temp gage, 2 low mod for front differential disable, PACBRAKE 5000# Air Bags with remote on board air...otherwise stock
1953 Chevy truck
496 2 NOS kits
383 2 NOS kits chassis car (street legal)//