The Hidden Forest

I know that the Alt-P quests have not been update in a considerable amount of time, but as a solo player, I was wondering if it was still a viable option to gear and level until ~45-50?

Do you mean viable as an alternative progression character only or as a way to level up and then switch back to normal progression (which I understand you can do as a one way trip)?

I've never done it but so far as I know it all still works, is good quality content (as limited as it is) and can be done solo to its completion. So if you mean the former, I think the answer is "yes". If you mean the latter I'd say "yes, but it's not the most efficient way to level up solo"

I believe it's a lot of fun and challenging so if you don't mind the fact that it's likely never going to be developed further and you like the look of it then it's worth trying out (you can always bail out as I said above)

Personally, it's never appealed to me because I love the server's primary content arc so much.

It'll get you up to those levels - but it will be slower since you won't have interaction with the existing community that is more than happy to buff the daylights out of you. Like Ydiss said, I've always preferred the regular levelling content.