Yeah, this game has more than one problem managing "narrative urgency".
Either you are told "YOU ARE GOING TO DIE IN 5 MINUTES" but then you can long-rest an indefinite amount of times (and you need to if you want to experience all the party dynamics), or you are told "YOU ARE GOING TO DIE IN 5 MINUTES" and then you really die because you made the rational call to rush to your salvation only to find an insurmountable fight.
As Tuco said, the fight isn't all that difficult but is just deadly if a player is going to face it as soon as it can (as the in-game lore suggests to do).
First off, nothing in game suggests that you are going to die in 5 minutes. You are reminded multiple times that ceramorphosis duration is about a week. And even Lae'zel tells you that symptoms you might suffer, become more severe the closer you are to your "expiration date", but however many times you rest you are showing none of those. Not to mention multiple times in game you are being told that those tadpoles are diffrent.
Secondly why trusting Lae'zel is a rational thing to do? She is unclear about what the purification might entail, she's alien to Faerun and the way you meet her after Nautoloid crash shows that she would be unable to survive on her own. For me it's clear that she is joining you because she needs you, not other way around. Furthermore, all other companions suggest finding Halsin as a better bet for cure. It takes a big lapse of logic to assume that doing what she wants as soon as possible is the way to go.
As for fight itself, it's not uncommon for cRPGs put fights like that as a barrier. I'd assume that whatever fights wait you on that path later on are even more difficult.
First off, those are some good points, but I'd like to counter them.
While you might not die 'in five minutes' and you are told that something is weird with your 'tadpole' everyone you meet still tells you "We don't know what is going on, you could still change at a moment's notice, you should get that out of you ASAP." And while Lae'zel isn't exactly the nicest person on the planet (or in the planes) she is the closest thing you have to an expert on mind flayers. Sure, she doesn't know Faerun, but she's been raised since birth to hunt ilithids and dialog even tells you that her people have access to a massive library the likes of which the planes have never seen. She is clearly the most knowledgable about mind flayers, and so the players are led to believe they can trust her on this issue. Up until the creche, everything she's said and done about mind flayers has been true. She knew the layout of the Nautalis, she knew how to activate the teleporter thing to get you to Faerun, she knows the capabilities of the Mind Flayers . . . she's been spot on about everything (except you turning, but then again, that has thrown EVERYONE for a loop.) Listen, just because Gale doesn't know squat about camping or Astarion doesn't know squat about surviving on the streets doesn't mean that they aren't experts in the weave or navigating the social circles of the upper crust. Same with Lae'zel: she's an expert on Mind Flayers (or as close as you have anyway)
But that's all besides the point. The game shouldn't lead you to an insurmountable fight unless it gives you plenty of warning. The Hag is a fine example of this. Every time you go after the hag, the fights get harder and harder and you get the distinct impression that the Hag is super powerful and hates you. You are given multiple chances to back off if you don't think you can handle her. That is fine. Having the game go "Hey, go here, see, this character even says it's okay. Look at all these cool lore drops we are giving you! HAHA You're dead!" is NOT cool and not fun.
Listen, if Lae'zel, before going in says "Wait, something is not right, he's not following protocol! He may have gone rogue, I suggest we leave now." then we wouldn't be having this conversation. If the Gith weren't as superpowered as they are so that a group of level 4s can't even take them down, then we wouldn't have this conversation. It's the fact that it is an insurmountable fight that we are not warned about and are, in fact, told it may be our best chance to live.
Now yes, there are some clues that things are not all on the up and up. The red dragon roasts some humans right before we go in, that's a problem. But we are assured that isn't OUR problem. Lae'zel assures us again and again that this is the best chance we have, and then even if you get cold feet she will fail her deception check 9 times out of 10 and the party still dies.
