Agree with most of your points and think Felix is going to be a bad signing for us but come on, it's nowhere near "Mbappe level of investment" seeing as Mbappe is on around €4M a week.
But yeah even if that cost comparison was terrible it's still a shit load of money for someone who's very likely not going to be a great fit.
Now that we're out of both domestic cups, before Felix is even eligible to play we'll only have 19 league games left plus however far we get in the UCL. All added up we're realistically looking at 21-23 games left in the season and chances are Felix is not going to play them all so at £15M total his cost per game is going to be very high. Even if he somehow fits in I still very much doubt he'll score more than 5-6 goals from now till the end of the season so is £2-3M/goal worth it? Feck no.
Only way I can see this becoming a good loan signing is
if he's actually great and we end up buying him on a permanent deal in the summer, but right now that seems unlikely. Or if he somehow fires us to a CL title which is even more unlikely.