Nera Lifeboon
Other allies in your party have +1 health.

Watcher Mal'wi
When an opposing ally enters play, Watcher Mal'wi deals 1 ranged damage to it.

Just wondering which will resolve first when an ally comes into play.

I don't think there's a contest here - allies come in with +1 health (it's a blanket passive effect) and then Mal'wi's effect triggers their coming in, dealing 1.

I would say that as soon as the character hits play, the blanket effect of nera is in effect, but Watcher Mal'wi has to go onto a chain after triggering.

Aka, Watcher has to wait for it's trigger, which means that Nera is already finished hitting play and is spreading the love.

Both guys are right. Nera is passive effect that does not go onto a chain of any sorts, its just there. So characters like Leeroy are safe from Mal'wi initially.