So, for those curious how this was resolved:
i called the LH First hotline, (very long wait times!) and they somehow had trouble confirming me for the flight with only 40 mins connection, which suited me perfectly. The system wouldn't confirm although it's a legal connection. Anyway, the agent offered the following alternative: LX F via ZRH! That's like striking the jackpot as far as I'm concerned as LX F is off limits (I'm not a member of Miles & More)
next question is the lounge access in ZRH as I've read many reports of LX denying F pax on their flights lounge access if booked on codeshare. (I know I'm not codeshare but just wanted to check to avoid bad surprises) Since I'm at ZRH right now waiting for another flight, I walked from the SEN lounge to the F lounge to ask personally if my situation would allow access. (LX flight number, F cabin, award on partner ticket stock) it took 3 lounge employees (including a supervisor) to confirm that as long as I had a boarding pass in LX F, I would be granted access. It seems that since a few months, they have abolished their very 'bünzli' rule of denying pax on LX F metal access just because they booked on a codeshare.
So all in all I think a very good resolution.
Thank you to all who participated in this discussion.