Thanks for the responses.

I was originally shying away from option 2 because I'd seen recent reports of OU being somewhat delay-prone, and ZAG being fairly disorganised. Couple that with needing to transfer from (intra Europe) international to domestic and it seemed a bit ambitious.

The Pula flights might well require immigration as well, but as a same plane service (OU's website listed it as a 'tech stop' only) it seemed there was no real risk of misconnecting.

As it happens, it's now a moot point as by searching on round trip flights rather than one way I was able to find a reasonably affordable connection on LH that let us fly the previous day, which was even better from my point of view. The joke will be on me however if we misconnect in MUC! :P