To the OP (and lack of reading posts between.)
I level VERY slowly, but play every day.
I don;t have to get a PT, by the time one forms, I would have to get off the game.
I'm on, off, on, off, constantly.
It took me from 1.19 to now, just to get from 15-25 in 1 class.
Which, compared to how I normally play MMOs, this is really slow. But it's ok.
The real problem lies in the manner you can level faster than that.
EXP groups.
If you raise the exp Needed, It just makes casuals unable to really ever play anything.
If you lower the exp gained from PTs, this helps, but no matter what, will always be faster. (And should be, otherwise what is the point?)

As for the claiming system, I'm sort of ok with it, but something needs altered.
It's kinda wonky.
The times I think it isn;t ok, is when you aggro 1 enemy in a group, and all of them are claimed (but some times this never happens)
If all are claimed, this makes PLing even easier/faster. (CNJ AoE spam)
For NMs, I think it's fine, but outside, I think they should be hit 1st.

And honestly, a PL who only heals a group, is JUST as effective as a PL who kills for you.
Groups kill enemies so fast, with a PL, that you wait on respawns.
But in a good group, who are trying, you kill the last enemy as the 1st repop, so in short, it takes the SAME amount of time.

In the scenarios where you see differently, you most likely had ppl leeching off the group with gimped gear, lack of effort, having bad combination of class/abilities etc. (to which a PL obviously makes better.)

But if you simply give me a PL who heals, while having a group who actually tries, then I wouldn't see any real difference. (maybe within a difference of 1-5% more exp, based on situational problems that arise.)

PLing is a bit of an exaggeration, BUT I do agree it IS the lazy way of doing things, and can prevent some players from getting the proper experience needed to play their class in groups, before endgame content.