What's the highest paragon level for a legit player ?

over 3000

First you would need to verify that the player is legit. Someone looking to convince people they don't hack could simply run the bot for 4-6 hours at a time, 5-7 times a week and fool everyone.

The difference between a no lifer and a bot is so small.

Can a legit player hit 10k paragon level ?

If they have an ample supply of hot pockets and a bucket to piss in they can do anything

no

could they - yes

would someone actually do so - no

It depends on what you mean by legit.

Something like turbohud could be considered "not legit" but it doesn't really help you get keys
What about abusing clan/stream viewers to get keys, so you just join game, grab key, leave? In a way, that's more effective than botting.

Gabby hit like 3.5k in 3 months and after 2k the xp is the same per lvl.....people will hit 10k I'm sure.

How long would take 5k-10k paragon lvl legit ?