Welcome to Gunco!
Looks like someone sold you a missed-matched parts kit with a virgin front trunnion. What kind of kit is it? Where did you get it?
If this helps any, I measured a couple of Romanian trunnions I have. The rivet holes are 4mm. (same size as the lower hole you have) You could probably use a 5/32" drill. Thats what I usually drill my receivers with for rivets.
A 10-32 thread calls for a #21 drill, .159" if you're using screws like you mentioned.
The rivet holes you are missing measuring from the front of the trunnion, are 10mm and 27mm from the front.
As far as drilling the trunnion, its softer and easier to drill than you would think.
On the barrel pin hole, what I would do is drill that after headspacing the barrel in. I would use a 1/4" center cutting end milll and drill about half way thru and finish with a drill bit, then ream to 7mm. THATS what I would do, there's probably some more ways to skin that cat.
I am sure some other more experienced members will chime in on this.