i agree with u/sarxsvt. it may be some non-silicone caulk or the wrong product entirely. the khaki colored stuff looks familiar to me but the product i'm thinking of has no flex to it at all. dries rock hard but i can't remember where i've seen it. just dremel the plastic nuts off and clean the area of any residue as it may cause issues for the new install.

Well I don't see what you mean about the metal header. I just see the wooden header and it appears to span the width of the original opening. As long as you don't plan to add a door back in the space I think it'll be okay.

That sounds amazing!! I wonder how much weight a leg sized leech could bear.

This is so stupid.