I have booked this itinerary several times, most recently in late April and middle May. I have never tried to switch to an earlier train so I cannot address that specific question. However, I did get 500 miles (including EQM) for all FRA/QKL segments.

I had been unable to secure a boarding pass for the train when I checked in for my UA flight, so after arriving in FRA, I asked an LH agent in the SEN lounge to print my ICE boarding pass. I was not in a particular hurry so I availed myself of a shower and some snacks while waiting in the lounge. I suppose that one could inquire about an earlier train at that point; it certainly does not hurt to ask. If turned down, you can also try again directly with DB, i.e. keep asking until you get the answer you want to hear.

Once on the train, there is usually a single car devoted to the LH "flight," typically car 21. Porters stay on board during the trip to assist passengers with luggage. On this particular trip, no one actually came around to examine my boarding pass but I know that you can't depend on that happening.

Incidentally, I once accidentally got on the wrong ICE train (booked directly with DB) from Cologne back to FRA and the conductor noticed it right away but decided to let it pass. I know this outcome is not especially likely in a region known for following the rules but it does indeed happen on occasion. Details about this interaction can be found in chapter three of the trip report, Fährting Around Frankfurt.