Regarding the phrase 破れないのが不思議なくらい指に力がこもっていた。


So from the context I expect to mean the same thing as
不思議なくらい指に力がこもっていたが、封筒は破れなかった/封筒が破れるにはならなかった。But in that case, what way 破れないの
is the subject, and does it have (withが) the meaning of なのに ?




The が that appears in this sentence doesn't mean "but"; it's the subject
marker.

破れないのが不思議だ = It is strange that it doesn't break.  
破れないのが不思議なくらい = So much that it's surprising it doesn't break.  
破れないのが不思議なくらい指に力がこもっていた = I was gripping it so hard that I'm surprised it
didn't break.

I think you have to treat な as a pre-noun form of the copula (see [this
question](https://japanese.stackexchange.com/questions/4879/relative-clauses-
without-verbs)). 破れないの is not the subject of こもっていた, but of a hidden copula.

