S.H.
He still seems to have his "startle reflex"... thus his arms are "jerky" as you say and he can't control it. This does wake them. Ask your Pediatrician about it and "when" they grow out of this.
2) since he is 5.5 months old, he is probably going to hit some developmental changes (6 months is typical for this), AND they go through growth spurts at this time more and need more intake and get hungrier... at this age, a baby still needs to be fed "on demand." But also, whenever they are about to hit milestones or developmental changes, they DO WAKE MORE. It's normal and just what happens. It's purely developmental.
3) try to consolidate his "naps" into just 2 naps a day... if possible, instead of 3. A nap of only 45 minutes isn't much. Maybe by consolidating his naps into 2 naps a day, he will have longer naps and more stable sleep at night.
4) why does he have to go to bed at 8:O0-8:15p.m.? You said he can't make it till then unless he has a catnap at about 5:00-5:30.... ? So maybe then adjust the time he goes to bed to earlier.
5) Yes, the more sleep a baby gets, the better they do sleep. Because if they are overtired, they will NOT sleep better and it causes them to wake more. BUT... perhaps he does not "need" the 3 naps everyday. Especially the cat-nap at 5:00.
6) A baby this age will NOT sleep ALL night... keep in mind that "sleeping through the night" means, for a baby, sleeping at least 6 hours straight. AND, their sleep patterns changes all the time... they are growing, changing, and their needs change all the time. At this young age, it cannot be expected that his sleep pattern will be the same everyday. Heck, even a 6 year old varies. Even a teenager has changes in their sleep pattern. Even adults have changes in their sleep. So... are they ALL "sleep challenged?" No, it's just that each child is different, and they all have different needs, and one "template" will not work for all kids. And you have to allow for their CONSTANT developmental changes.
And, it could ALSO be teething too. This also wakes them from sleep. Or gas, or at this age, they ALSO start to develop "separation anxiety." It's normal. At EACH age, there is "separation anxiety" and it gets manifested in different ways, per their age.
A baby's sleep will not be the same all the time. Sorry. I have 2 kids... and per their development, there were periods where they would wake even every hour for cluster-feeding or teething, or gas, or just because. Then the "phase" goes back to normal. It passes. You just have to accept it.
My daughter is now 6 years old, she did not "sleep through the night" until she was about 2.5 years old, at least. My son is 28 months old, and he still wakes about 1 time a night usually. But by now, well that is no big deal.
Your baby is only 5.5 months old. Just see what the Doctor says... but it's all pretty normal stuff.
Al the best,
Susan