2 things...

[-DMR-]

Ok, first, I'm wondering why 3 of the last 10 level requirements in the 2.0 storyline weren't there (lv45 and 47/8) Anyway, I'm levelling my alt to 48 for this purpose.

Also, in the storyline up to 2.55 from 2.0, is it right that I can freely level up from 50? I was on 50 throughout, up until Heavensward on my main ages ago. What about the gear level requirements in certain battles up to the end of the 2.55 area of the story?

[Noxifer]

As long as you have HW registered on your account, you can level up to 60 -- it's not locked behind MSQ. Level 50 MSQ quests also give exp now, so yeah. You're likely to get at least a few levels from that and roulettes.
And since 3.0, they adjusted some of the 2.1+ MSQ quests so they give out gear, enough to bring you up to par to the duties required by the MSQ. (So 2.1-2.2 will give i70, 2.3-2.4 i90, and 2.5 i110, if my memory serves me.)

[Canadane]

First) To get you to do some side quests or redo dungeons to gear up.

Second) As long as you have the expansion, you can level up to level 50 well before reaching HW content. Likewise you can gear up as much as you possibly can acquire gear from your location, including the market board.