Regarding the apparent lack of top speed from Mercedes powered cars, a theory I've not read yet is the possibility that Mercedes may have deliberately gone for a smaller and more efficient PU, akin to what Renault did to great success in the second half of the V8 era (particularly with Red Bull, but also with the former Renault/Lotus customer team).
Obviously if that is the case it's not bearing fruit yet, but it could open up huge aero possibilities if Mercedes can get on top of their "min pod" design during this locked PU era, as no other PU manufacturer will be able to adopt such an approach until 2026.
I know there have been some departures from HPP and challenges getting the most out of the E10 fuel for all the manufacturers, but I can't imagine they've been left behind to such an extent when they finished last year with the clear best PU (especially out of the box).
It's just a theory, but in light of just how compact the Mercedes is and their relative drop in power, I do wonder if it's a case of knowing they would lose their power title but thinking it can be made up by gains elsewhere as a result.