1.Is alduin the strongest being when he is eating the world because no gods try to stop him at all?

2.Is alduin a dragon when he its the world or does he turn into something else to eat it?

1 - Of course the gods try to stop him, they all try to, he just eats them too. Two gods (if you can call them that back then) certainly tried to with a 'clever' plan, it didn't work out well for either of them.

2 - He starts as an aspect (or not):

Big Dragon > Eat some souls in Sovngarde > Bigger Dragon > Eat more stuff > Bigger and more powerful....(skip a few steps)....> Finish eating Mundus > Eat Oblivion > Eat Aetherius > Make sure he's eaten everything > Once there is nothing else to eat, eat himself > Next Kalpa.

Akatosh and Shor managed to stop him this time around.

I assumed that they only delayed him. Stopping him entirely is a short-sighted plan as so many things can go wrong with the stability of Mundus (Thalmor's plan for example), and removing the in-built reset function doesn't seem a spectacularly good plan.

Of course, Alduin may have been already derailed after Felldir shattered time and hurled him into the fourth era. This Kalpa may have been extended way past its due date.

Delayed him but it shows that they're enjoying this Kalpa.