Should i unlock my Nino(-atk/+spd) to 5 star

Nino is much less dependant on their base attack than other mages, thanks to Gronnblade. Sure, -Atk is a pain, but +Spd is probably her best boon. The issue with her is not so much her overall strength - but that she needs team support to really shine, so you might need to change your team a bit.

As for replacement - I'm not really fond of healers, so personally I'd trade Clarine out, but it depends on your playstyle - you can always try to inherit Breath of life (from free Felicia) if you require healing. That said, -Atk Robin is hard to stomach, so if you'd prefer to keep clarine around, that's a good option as well, even at the expense of losing effectiveness vs grey units. Nino also pairs better with your effie.

But since Nina is so common, it might not be bad to just wait for another one (she can show up as a 3, 4 star)