When you put the new coils in did you grease up the coil boots? I replaced all mine and didn’t have any boot grease and thought it would be ok. It wasn’t. Caused all sorts of misfires.

to me the rusty cylinder really does sound like either a blown head gasket or a cracked head in the water jacket.

 

a blown head gasket or cracked head would explain the problems you are experiencing. Leak down test would be good idea.
For my education, I'd like to hear about the broken coils. Where were they broken? Any idea what caused them to break?

 

Sounds like you dealt with a dishonest seller who wanted to unload a vehicle.

Look up videos on block testers and how to use one. Best method I know to test for blown head gasket. Normally I would tell you to lay all the plugs you removed side by side. The one that looks steamed cleaned would point to a HG leak for that cylinder. But you've got a broken plug and penetrant so, so much for that.

I once had a small HG leak that I fixed 'chemically' many years back. Not the preferred method, but it worked and cost me about 20 bucks. Lol. I drive it daily today.