you nailed the short list.

any others?

no. not that are a direct bolt on application. there is the ar15 telestock model, with the adapter block. as you know, that is not a folder.

Are mods needed for the East German wire side folding stock?

no. just be certain to get the oval head countersink style metric machine screws that affix the east german and roamanian wire type right-side folders to the existing rear receiver trunnion.

From the grainy pictures I have seen of the East German side
folder it looks like it has it's own proprietary rear trunion
and thus the receiver would need extensive mods.

the east german and romanian models attach to a standard akm rear trunnion, but the east german lever actuated models are very rare in the usa. 99.9% of the models imported are the romanian push-button versions.

As I understand it, real military Krink side folders need
extensive receiver modifications to attach (remove rear trunion,
mill + drill).

yes. to do the job correctly take several hours the first time thru and requires a specific rear trunnion be riveted in place of the old akm trunnion.

Underfolders need even more mods and are not
especially stable, so I'm not interested in them.

the mods to the receiver are definitely required. as they age from use, the underfolders lose stiffness. they also offer one of the most uncomfortable shooting positions i've expirienced. like all metal buttstocks, they are cold in the winter and hot on warm, sunny days.