Well, looks like you have found the problem... loose wire will do it....
As a matter of course, I replace the tlp350's, the 4 totem pole surface transistors on that board too., ALL the fets and all the fet resistors ( the through hole ones, not the 20k surface mount ones)
I have not worked on the later model ones with a daughter driver board...looks like it makes it interesting.
On the 48v units there is a surge diode that can go , it is obvious which diode that is, it is an add on under the control board... may not be on the 12v one... as it is a 12v surge diode.....
Check the zeners on the floating power supply drive for the tlp350 chips, and you should be good to go.
At least we know what got it.... like I said, they are very very hard to kill when they are running properly as a straight inverter..... I have not been able to thus far.
Disappointing your relationship with the seller stinks.... that is a true workmanship warranty, and they are generally very good with that..... you must have pissed them off bad.
I have never claimed warranty, as I know why I blow things up... but there are a few that have, and been very well looked after.
You could try to kiss and make up, or at least get them to sell you a new control card.. around $45 plus shipping, last I got one. The main board is easily fixable.
Sometimes digging your heels in is not the best way forward.
EDIT: Just looked at your pics again... you do not have through hole resistors on the power card.... so you will need 24x 46r or whatever they are surface mounts