You can lvl all jobs on one character but if u just like making alts for each job then it's pretty much the same as it's been.

No the aether currents are not unlocked account wide, u gotta do them on each alt u have.

When I bought a jump option for my alt, story jump option that is, back in stormblood then it unlocked all the aether currents accept the ones in the stormblood areas, bit were jump potions go to 70 and u can jump to shadowbringers I'd say a story jump option to SHB would unlock all aether currents except the shadow bringers zones.