To complicate your problem, the EME standoffs are probably threaded 5mm .8 rather than the 10-32 you suggest they are. 10-32 bolts thread into 5mm threads just fine but they are loose by several thousands of an inch. Loose enough to cause problems in many cases. Please check to see whether they are 5mm or not.

Why don't you just punch the blind nuts out of the firewall and in their place use the correct size socket heat cap screws threaded in from the back. Use adequate large fender washers on the front and back side of the firewall to distribute the load and you're good to go.

This is fairly standard practice. In fact, it is rare to have blind nuts in a firewall and I wouldn't use them if they were there. Not strong enough in my opinion.