Another question... Helena or nursery?

Well it really depends on what you need. Helena is a decent support with bad cool downs, while Nursery is a crit-based caster.
As someone who uses Helena a lot I can say that she is a great support for farming or just trying to clear nodes in three turns.
If what you need is an offensive AOE caster than you have the better option already, Eli Caster, as I'm assuming you have her at NP5.
If you want to mess around with crit teams or NP spam than Nursery is your girl.
I hope this helps.

Helena if you in need of a support. She's the closest thing we can get for a quick support for now.
Nursery is a great np spam and boss killer. Raise her if you need a ST farming caster.

Helena is more famous for her supportive role. There was no such servants can provide art-quick-buster triple buff to team. Her issue is her CD for skills is way tooooo long, then as a supportive servant, normally her role is to provide your DPS maximum support in a few turns, finish the battle as Blitzkrieg, particularly in a critical team. You can try her with Lancelot+Hans versus lancer, or with Liz + Hans versus Assassin.
She indeed has AOE NP but she NP damage isnt that high (especially right now), her NP is capable to provide debuff to enemy which shown more tactical function than simply pouring damage.

Nursery on the other hand has more restrict function: kill the enemy. She could be one best offensive caster because of her 1st-3rd skills are top-tier liked, but the critical issue is: she require other servant to send stars to her, since herself cannot easily generate enough star. Meanwhile Art-critical team still wont be paired with Buster critical team to deal damage. Therefore, imo, Nursery is really good as offensive caster but also really required good team composition.

It is not a bad idea to have Helena+Nursery+Hans in team against assassins, but if you dont have 2030s, Waver and Tamamo, then Liz caster can more or less replace the role of Nursery.