Adding 100 points to EXP for Apostolia stat?

Only you can answer that. It adds 100% Exp (not sure how it stacks with books and tickets)
If you have Stelfy and you want to level cap, then go for it.

I think he meant exp not hp

Yes it helps very much, but yep it depends on you if you can sacrifice those stat pts to gain more exp, and aside from EXP stat there's also IgT and IgR which helps you to recover and prolong the duration of Ignition buff, when you're at ignition apostolia stat Exp gain is boosted they do also helps. Here's a detailed formula for those stat;

EXP:

1 stat pt = +1% EXP gain
100 stat pt = +100% EXP gain

IgR:

1 Stat pt = -1 sec on recovery duration
100 Stat pt = -100 sec on recovery duration

Note: To recover 1sec in 0 IgR you need to wait for 180 sec to recover 1 sec, to recover 30 sec you need to wait for 1 hr and 30 min
if your IgR is at 100 you only need to wait 80 sec to recover 1sec, to recover 30 sec you need to wait 40 min

IgT:

1 stat pt = +1% duration on Ignition
100 stat pt = +100% duration on Ignition

lets say you activate your Ignition with 30 sec recovered time, and 300 sec recovered time

duration if 0 IgT = 30 sec, 300 sec/5min
duration if 100 IgT = 60 sec, 600 sec/10min

Here's my own experience, I've been playing for about a year and 5 months, already did 3800+ Ignition activation, those counts unlocks pages for apostolia as well;
500 activation - 2nd page
2000 activation - 3rd page
already at cap but it took me over a year, I could've done it faster but I'm always busy doing my prod, farming mats..

Once you achieved your desired whatsoever (lvl cap and/or apos stat all cap and/or unlocked 2 or 3 or 4 pages) then used a stefly by then

Exp spec stat does increase the exp you get from mobs, ignition increases the amount of exp stats get when raising them

It does it's like having 100% exp book on at all times