First: Yay! I love AD&D style initiative. 3E's initiative was my biggest disappointment with the system. It made combat downright boring compared to previous editions.
Second: I don't have 5E yet, so I can only speak to how I've played it previously.
Third: The DM should decide at least roughly what he's going to do before player declarations, to be fair. The players usually discuss their actions before making declarations, so as long as they're acting cooperatively, it doesn't matter who declares when.
Fourth: My group handled it differently. We used 1d10 ascending for initiative. Each player and the DM for each group of monsters declared actions on their modified initiative (roll + speed for weapons, just roll for spells) Spells started on initiative and completed casting time later, and were interruptible in the interim. If someone changed their mind about what they wanted to do, that was fine, they just used the new initiative. If the new initiative had already passed, they went immediately. Initiative is the time of first attack, each iterative attack happened one at a time with a delay of weapon speed. It sounds slightly complicated, but once you're used to it, it plays very fast.
The biggest difference, at least the way we played it, is that casters were extremely vulnerable during casting. No movement, dex bonus, dodging, etc during casting, and if hit they lost the spell. A lot of strategy was based on keeping enemies off the casters while they brought the hammer down.