[quietvoid]

I haven't searched for it yet, but it would be nice to have two sample files that have the same content, where one is encoded with HDR10 and one with DV and where you can actually see a difference between the two. Does anyone know of such a thing?

The Spears & Munsil UHD Benchmark has a lot of differently encoded HDR streams, from 600-10000 nits HDR10 to MEL/FEL Dolby Vision.
I've used it and it's obvious the difference the dynamic metadata makes in the case of the 10k nits version compared to my LG C8's HDR10 curve.

3. Check if the devices DV decoder supports the DV profile and level. (possible to check if the profile is supported on the Shield at least, I don't have any other devices)

Didn't you say already that the Shield doesn't report it supports profile 7? Most devices don't signal their true capabilities.
Making it a strict condition would result in all profile 7 files to fallback to HDR10 only, which isn't really the goal.

The mkv support for P4/P5/P8.1 profiles can be done later.

There really is not much difference between the profiles, the same RPU metadata has to be sent correctly to be used by the decoder.
I don't see a problem in supporting all of them at once, there should not be anything "special" required specifically for each profile.