No, very standard

Generally if you do the flight number swap soon after booking, the same fare class on LH is available and the swap works.

Not sure if the fully refundable as a twist but the same fare class is standard, long standing requirement.

As for buying from LH, as the TATL fares are joint ventures routes, LH and UA share the revenue regardless who is the flight operator.

Non JV flights, revenue goes to the flight operator regardless of the ticket stock.

So, it makes little difference to UA which way you book if the flight operator stays the same

The big difference is how the PQPs are calculated.
And for GS the LH operated flights, regardless of ticket stock, are not counted.