Time travelling / "repeating" the final battle

Hi there! I'm currently DMing a short campaign (the idea is 8 sessions or so but we have 8 hour-ish sessions with a lunch break) set in the Underdark. I'm not really a newbie DM but I'm not super experienced either, and I have an idea I wanna check with y'all because I'm not sure if it's cool or just lame so I'm asking my fellow DMs.

Long story short, the BBEG is a regular guy (even with the commoner statblock) obsessed with monsters and tries to make a super-powered creature to use as a weapon during the war with the kingdom on the surface (lots of homebrew lore that isn't really relevant other than the Underdark nation is actively fighting the surface kingdom). So, the final battle of the campaign will be stopping this monster, and here comes my idea.

The battle starts with the BBEG injecting the monster with a "super serum" or something like that to make it incredibly powerful, and it TPKs the party and its allies. It's a really violent and unfair fight. When they're almost wiped out, the magic item a PC has been carrying around (an amulet from the goddess of time, but they don't know its effects) activates and the goddess of time gives like a short speech and lets the PCs basically rewind time a few minutes, giving them enough time to stop the BBEG from injecting the monster with the serum, and initiating the regular -and not completely lethal, even if difficult- boss fight.

My main inspiration was the Twilight battle on the final film (yes. I know it's cringe) where the battle plays out and later it's revealed it was just a character seeing the future, and they avoid the battle. But, we all know D&D isn't a film, and I don't know if this would be as cool to play as it is in my head. Of course, the "fake" battle would be as quick as possible to avoid making the players feel like they wasted their time, just to make them fear they will lose for real without turning into a slog. Also, my players are really narrative-first and I usually use the rule of cool when it makes sense, so they value more a cool story than mechanics, but I'm still unsure.

So, do you think this is doable, and most of all, enjoyable?

Maybe I''m just no fun, but I don't think time travel works especially well in an unscripted medium where story beats are determined by random chance. There's also a key problem here, which is what happens if the PC's bungle all their rolls to stop the BBEG from injecting the monster? You're then stuck either obviously nerfing the monster for no reason, or needing another mulligan.

 "It was all just a dream" works for a one-on-one story beat, but I don't think having a big stressful battle, then saying "just kidding, now do it for real" would be effective. This is related to the reason why fiction does not painstakingly explain all the steps of a plan, then you watch the plan happen successfully on-screen. Plans are either NEVER explained fully, or else something unexpected happens to derail the careful plan. If there are no surprises, the plan becomes a spoiler and boring.

I'm also quite confused by your plan. You want to rewind time a few minutes so you can stop the bad guy from injecting the serum, but he has, for some strange reason, commoner stats. As in he'll go down in a single round from a common housecat. That sounds very anti-climactic and unsatisfying no matter how much he relies on monsters. I have seen a few instances where major named villains in fiction, with lots of build-up crumble at practically the first sign of resistance from the hero. It's always unsatisfying, and I don't think it'll be any different for your players. He should be able to survive at least one full round of attacks from each of your players. As it is now, he'll be dead before most of them get their first turn. I would strongly rethink that.

I think it's an interesting premise, but railroading PCs down that likely won't go well. 

First off: make it a difficult fight, not impossible. You can, for sure, kill off some likeable NPCs and make the party angry, but give them a chance to defeat the monster. PCs love unexpected victories.

Secondly: Give the guy some actual stats (even if it's just apprentice mage). Imagine that you prep this whole battle, the PCs are pumped, he's about to inject the monsters, then the wizard firebolts him for 4 damage. And he dies. Nobody is going to enjoy that ending, not even the players.

I love this idea of time jumping, but give them a little foreshadowing on what it does so it's not totally random.

The BBEG in question is actually not really a bad guy actually, but I used the term as it was the fastest way to convey the message. The guy is an NPC who is actually not a villain, and while his reasoning is flawed, it's not actually wrong (and actually, the PCs could just let the plan succeed and avoid killing the monster, but I don't think they'll like the implication of the monster killing tons of innocent civilians even if they're on the other side of the war). Killing him is actually another difficult decission since he's not bad, just somebody who is fed up with the war and designed a really nuclear plan — part of the idea of giving him commoner stats is to make it harder for the party to keep the guy alive, but I still have to give it some thoughts since this is just and idea still hahaha.

I'll give it some thoughts! As I said on the other reply, the BBEG is not actually a villain but a likeable NPC, so actually killing him would also be an interesting story beat. Of course if they tried to stop him without killing him they can try, but being so easy to kill is also part of the plan — it's a moral dilemma for the players, one life in exchange of many. I could maybe make the time jump faster (like after the first round or the first two) so it doesn't feel like a waste of time. The most I'm afraid of is coming off as railroady to my players, but to be fair, as a DM I have the power to decide the BBEG, the final boss fight and it's stats, "scripting" the first round of combat doesn't feel much different from people making encounters at low levels where the BBEG wipes the party and flees or whatever. But I'm also aware it relates heavily to each DM personal style, so I love seeing how others see this.