Is it still possible to hit top 1,000 in Arena Assault without heavily merges Legendary Heroes?

Umm, maybe? I suppose you can.

No.

Yes. At least I think so anyway.
You really only NEED (not want) one copy of 6 Legendary Heroes, and 1 merged Legendary Hero.
Because Arena Assault scoring is determined based off of your starting team, you only really need one team of maxed out +10s, with one Legendary Hero for boosts & the +1 bonus.
If you can beat enemy +10 units with unmerged units, you don't exactly need heavily merged Legendary Heroes, just some Legendary Heroes.
I could be wrong, but this is how I understand it.

Depends on your team. If you have 3x +10 armors and ONE copy of L! Hector, you might be able to.

What I can tell you is that on my run from yesterday, every single team had a +10 L! Hector, and half of them also had a +10 L! Ephraim.

For what is worth, I hit rank 245 (5280) yesterday with a starting team of Zelgius (+8), Effie (+10), Sheena (+10) and L! Hector (+10).