In both scenarios, he's a dead man.
I am not sure.
The huns already desintegrated more than 2 roman armies on the balkan. Roman armies did just not work against the huns. That is at least what the romans thought after these terrible defeats.
Cavalry armies, especially light cavalry are indeed near to invicible, if you try to catch them. They simply run, if things go wrong. On the other hand, they are not that good in defeating an army, which is holding a position. So Attilas challenge would have been to destroy these roman armies on the Balkan first, before he sieges Constantinople. I am not sure, if that was impossible.
If Attila is catched by an roman army while sieging Constantinople, he is toast. No question about that. But I am afraid Attila knew that.
I have no doubt about the siege itself, because the Huns were pretty experienced in sieges. While the germans were lousy before they learned from the huns.
Regarding ships and the Huns. Nobody thought, that the Vandals would ever have ships. But they did it. And the Huns, at this point of time already had an empire with a lot of other tribes. They simply could have used a gothic fleet. I am not sure, that they could not have bypassed Constantinople. And afterwards? Who should stop them while raiding Asia, Syria, heck even down to Egypt? The magister orientalis? Honestly?
I repeat my opinion: without the gold of the ERE, history would have been different in the east.
PS: What I never understood with all thes steppe-tribe-armies from huns to mongols: Somewhere must be a kind of camp, with women, kids and all the nice loot and supplies. In the case of Attila somewhere in Pannonia. If you attack this point, they must defend. And light cavalry armies are terrible in defending a fixed point. Well, when Attila reached Pannonia, it was no pure cavalry army anymore.