First, in most cases he does not copy the abilities. That being said, he already has a ton anyways so outside of very specific powers like GL’s ring constructs or zatanna’s spells, he either already has it or can convincing use his own powers to simulate it.
Now to your questions
1. As far as I’ve seen, no, there is no set limitation on replication or duration. That being said, most if not all powers and abilities at some point will become exhausting.
Ex: Supes can use his laser vision or GL can make a construct but for prolonged periods it likely would be hard to maintain.
2. For artistic reasons it’s probably easier to draw a default, ctrl+c ctrl+v. For in universe reason I believe, and I could be wrong, it’s because the Martian society as a whole don’t put too much stock into physical appearance because they are mentally oriented as a society and as beings.
Also if I can find it I think there’s a MMH run that touches on the topic of a martian’s physical appearance.
3. Yes, and this can be seen in one of the dc animated shows. He flips through tv channels and shape-shifts to match whoever is on the screen. Granted that’s video footage not a photo, but he was switching channels quickly.
- to expand that question I assume you’re asking “can he shape shift into something from a photo- and be 100% accurate” which the answer is likely no.
-Now does he need to be 100% accurate? Depends on the mission. Into the average goon? He can skimp out on extra moles and scars. Into a known villain? Yeah maybe get the colour of the eyes right.
-With that said the events where MMH has to shape-shift he’ll likely have the physical referent to copy or videos/photos of it from multiple povs .
Been thinking about Martian physiology (disclaimer: limited knowledge of both it and the entirety of Martian Manhunter canon).
While there's a debate and conflicting canonical mentions of whether MM can take on powers and abilities, one definite capability is the way he can transform into anyone (and anything)
Which leads me to a few questions:
Are there any limitations to transformation duration/anything he can't replicate?
Why do all Martians look the same/have a default?
Can MM transform into something based on a picture? (As in a never before seen, completely novel entity? Since he's transformed into humans before, I'll say he can transform into any human based on an image)
If we apply some principles of human biology to them, Martians are kind of like B cells. They can hypermutate antibodies to even capture/get close to disease states never before experienced. Martians probably possess massive genomes with lots of splicing and recombination to be able to recapture all the possible appearances. I'm assuming they're cuttlefish like.