To report back on this, yes I had to cancel the flight on the day of departure, yes the LH website didn't allow me to cancel it (cancel button was just gone), so I called in and the agent promptly canceled and refunded the ticket without any issues about 4 hours before departure time.

I didn't check in and I didn't select a "complimentary" seat beforehand just not to complicate anything.

Again this was on a intra-European P11BUZE9 fare.

So all ended well. I may be too paranoid with LH but they treated me pretty well this time (also had some business class flights with them that I did fly), not sure if related to my high-fare tickets, *G status or an actual general improvement in customer service. The only thing to note was the LH Welcome Lounge agent in FRA sneering at my UA TATL F ticket, indicating that LH was doing me and UA a "favor" to let me in and that I shouldn't have any further expectations .