Since これ comes before で in those examples, how come the これ becomes postpositional complement?What do you mean by not having a specific function?! Even if the de is what you say, it does have a function. But it is a postposition, I think.
Before determining what it is, we need to have a look at similar expressions. Examples below all take これ for a postpositional complement:
で正しい , で正解だ, でいい, and slightly related で合格だ