I'm sure the replacement of the new motor will be handled just fine, it always was with the old motor, can't fault the warranty support. it's just a bit of a pain in the backside swapping motors so frequently, I was hoping tq might have improved the reliability on the hpr60, but it not looking good on that front so far.
At this stage and with your history, I'd argue that isn't the point. You can't be
this unlucky - either you are doing something to shorten the motor's lifespan, or there is a problem with the bike that the motors are going in. Four motors in a short-ish span of time suggests something
really bloody weird is going on unless someone at TQ has it in for you specifically.
On the warranty front, as expected Trek do state the warranty only runs from the original purchase date, which is why issues like yours should be tackled from a Consumer Rights angle, not the warranty itself.
With your ongoing issues, you have far stronger backing from the CRA 2015 than you do from Trek's warranty. With the HPR60 suffering the same fate as the Trek-supplied motors, you could argue that there is evidence the
bike itself is the common factor to the failures and go at Trek with that, if not go at TQ themselves with your newly-purchased HPR60.
Ultimately you will do what you feel is right, and that's okay. I'm approaching this situation from the standpoint of being a cautionary tale for other people with similar ongoing/recurring problems. Argue from the angle of your Consumer Rights if you are in the UK,
not from the frame manufacturer's warranty.