Rolling for my team.

If there is a rate up for Atalante coming in the future I am unfortunately unaware of it. The closest I would assume to be the singularity rate up summons that come before a chapter release, but I looked it up and she doesn't seem to be in those either.

As for your other question, I must say that its a bit of a tricky one primarily for the reason that neither of these two servants are really geared towards your setup. Both are strong for sure, and both would bring a lot to the table, but you have the makings of an ideal quick based crit spamming team right there, and yet neither Jalter nor Cu Alter have anything more than one crit card in their set up; however, if it is really between the two of them then what we have to look at is not who will contribute more to your Okita Jack combo, but who will benefit most from it.

Even looking at it this way, however, the answer is muddy. To begin, Cu Alter is a berserker and that means a low crit gather rate. Of course with Jack and Okita you may still be able to regularly produce enough crit stars to give him high percentage, Berserkers in general just don't get very high star gather rate, which generally makes him less viable in a crit deck. It can work, and on the turns in which you find yourself with 40+ crit stars you can use him to deal massive damage, but it's still a strat that may be lacking.

However, Avengers aren't much better than Berserkers when it comes to this issue. In fact, after Berserkers, Avengers are the 2nd worst class at crit star gathering. Jalter does however have something in her favor that Cu Alter lacks, a skill that compromises for this weakness. Her Self Modification skill (which is available even at base ascension) greatly increases her crit star gather rate for three turns as well as the damage done by a critical hit. Of course, this skill has a cooldown, but if you can max it out to level 10, the cooldown is only five turns. Meaning that for 3/5 of the time, you will have an absolute monster able to consume all of the crit stars produced by Jack and Okita before dropping them down for massive damage.

Ultimately, they are damage dealing focused servants that wont really be generating you that many crit stars, and usually wont be getting the benefits of what crit stars you do generate either; however, with the addition of Jalter's base level crit absorb skill she has the potential to be an absolute monster in you set up.

First thing first, I'm assuming you are trying to build crit team with Jalter or Cu Alter as your hitter.

If that's truly the case, Jalter. Give her CE like Teacher and I or Self Designated Enforcer, you are gucci

Cu Alter's star absorb is too low to make use of those CEs well, especially when you pair him with the likes of Okita and Jack. Even SDE would only give him at most big enough star absorbs to be on par with both of them.

Jalter is a different case. Her base star absorb is decent enough to make her monopolize the star with SDE + her first skill. She will abuse the stars more than you'd think despite being an Avenger.

About Atalante, sadly I didn't pay too much attention on her, I'm sorry. You can just summon her on singularity banner when they come out though.

Atlante never got on a banner in the NA till now, even before a new singularity.

And if I have to choose between Cu Alter and Jalter, I would go for Cu Alter because he is berserker (effective againt all class), has great survivability (Protection from Arrows + Guts) and one of the best Atk stats on the game till now. Plus with Okita and Jack, you can generate a tons of stars and he will always a big % to land critical blows.

Jalter is an avenger so she is effective only against Rulers (only 2 servants in the NA for now) but with the stars generated by Okira/Jack and her first skill, she is a great Critical damage dealer.