No. Iirc, more NP only increase heals per turn, so it's not worth it considering the SQ and probably you have another target.

NP2 only gets you an extra 500HP. There might be a rare as heck scenario where that extra 500HP will save you, but gambling a shot at Castoria is not worth it.

She'll get a banner in about two weeks, so spend accordingly, I suppose.

I'm already prepared, but nervous, I reached the Pity Summon limit, but I now have 949 SQ and 21 Summon tickets.

It's my 3rd attempt and I can never fail in getting Castoria.

Well aside from that, there is also the perk of having the NP gauge not being capped at 100%, making it so that an NP drain may not outright prevent an NP from launching.

I am still unsure whether this and also the aforementioned heals per turn is significant enough to warrant an NP upgrade

Definitely not. A Merlin Castoria support combo is almost always satisfactory with both at NP1. There's virtually no scenario where the extra NP levels will be worth it strictly from a gameplay perspective. The only time the NP gauge situation you mentioned will always be relevant is the full power Kiara fight, and if you want to do that with NP1 servants you just buy the thing that gets rid of the drain. Otherwise if you fail a fight and that heal ends up barely being too low, just try the fight again instead of potentially using all of your quartz for another NP level or even failing to get another copy.

True, but how many times the enemy has NP drain? From my experience, it's really, really rare.

It depends on your expectation of him.

Merlin NP 1 is fine when you're not expecting him to stay on field for long time.

Using him just to buff the Buster DPS, then he left the field from using Taunt, like that.

If you wanted Merlin to stay, then NP 2 is better.

I think people forgot its not simply heal 1000, its heal 1000 for 5 turns in NP 1.

Proto Merlin HP Up only lasts for 3 turns; Merlin's is far easier to stack when you can't finish the fight fast.

1.5k heal for 5 Turns is a good deal even by JP standard, not to mention modern AOE Buster DPS like Kukulkan, Richard, Metatron; all good in helping you keep listening to story of the king that 4.5k heal + 15 stars is a realistic expectation.

About Castoria ... I personally not recommended her unless you leaned to Arts.

JP is at point of 'red, blue and green each have their own method' with the 90++ and gimmick they got, so as time goes Castoria wouldn't be the problem solver for 95% of stage again unless your best hitters are Arts.

Well, it will not be his last rateup ever. Save for Castoria, use Memelin a bit and then decide to go for more NP levels according to your future Servants targets.

And get a friend with a high NP level Merlin for testing.

From my point of view more NP levels of Memelin are always useful if u are into turtle battles.