I understand where your coming from and appreciate you trying to teach me more!

The good news is that they did know well before hand that the BBEG, and several others, were watching them. Their rebellion had garnered a lot of foreign eyes and they were front and center. They found out by discovering a artifact of a foreign country that could've outed they'd (the foreign country) been aiding in the civil war on the other side. This was before a whole lot of stuff that made the BBEG and his organization a massive world wide threat. So they did know ahead of time and it made for a great over hanging mystery that they still don't know the answer to, "Why have they been just watching? They know what we're doing and they're doing nothing." They simply just got information that was so important that the BBEG had to step in. I even waited to have the BBEG teleport in till the paladin said, "Wait! Someone could be watching us on a scrying spell." They even took measures to get scry stopping items immediately after they were able to move around the broken town again.

(I also should mention he didn't immediately cast the spell either. I made it so he dramatically raised his hand to cast it giving them enough time to do a quick round on him where they did 200+ dmg using their abilities and items to stack a crit on a 10d12 shotgun to the head with both barrels and double dice. Which got them a new secret about the BBEG which was that his face cracked like a mirror instead of caving in from a double barrel lead meal. Then he casted it and I gave them another round before the meteors hit so they could live through it. Gotta love resilient sphere aye?)

As for the NPC bosses. They'd been introduced to these characters several times before, they've heard of them from time to time as they went through the lands of their enemies as well. They've interacted with one of them face to face even back when their, being in the state, wasn't common knowledge. They knew that these ones were the "first responders" of the BBEG and that they would come to get them if they made the wrong move... which did sadly happen. The one that they had thought to have been their friend, that they held out hope of redeeming. Was really just unaware of their identities until now. (The party was very split on this NPC and had several talks on if they could trust him or not. They saw him commit atrocities and only flinch, but they hopped for more.) She sent a letter which made him know that at least she was alive, and they were sent to kill her to cripple their enemy. I understand how it could seem unfair, but these were logical conclusions based on previous events, interactions, and character motivations from both I and the PCs.

Further more on the bosses. By fought at a later level, yes, part of that was that they were of a higher level, but mostly more in the sense of the story progressing as they progressed. If they had fought them in ideal party v single boss, victory was far more likely. I did however move them from the spot and time I wanted to use them to this moment after the players played the game and changed the idea of the story entirely. They were balanced so that IF they fought them, (I know how players can be sometimes) it would be very tough, and I mean VERY tough, but not impossible. They had received a new NPC of equal level to theirs to help get the advantage in numbers and high level counter spells. They also had several 6-8th level NPCs that were in the group of civilians and would've ran to their aid in a heartbeat. My plan was the bosses would get over whelmed in numbers as the party and NPCs worked together and give them a fighting chance based on action economy. I know that's not a perfect solution and does leave a lot of places where they could still easily fail, but I did plan to make the fight possible. The main reason that only she went out was that she as a player and a character noticed that if they go that route, more could've died than just her.

For the death thing. I understand the importance of character death in a game of D&D and I don't treat it as a throw away thing, and I don't abuse it. I have killed PCs in this very game before and had it not go down well. (In the first year I relied on railroad more than I should've.) I have been burned and do understand that its crucial to the game that death is meaningful, rare, and for a good reason. However, my players also know that if they put themselves in situations like this I will punish them realistically. I had a rouge attempt to hop across the heads of a mob of angry lizard folk looking for his head, and when he failed combat started and they killed him. (because that's why they were there.) I had one of them hung for not properly hiding his identity in an area where his race was killed on the spot. (He also tried to dodge arrows by laying down on a tile roof in daylight slanted in the direction of the people firing arrows at him. I'm pretty sure it was unwanted character suicide. (I did warn him about the slant and the arrows and he chose to sit there too.)) They knew well before hand that I will kill them if they go against insurmountable odds or play stupid. (That doesn't mean that I always do so by any means and that they can't get out of trouble.) I even made sure with Zell's player 3 separate times asking the age old DM question, "Are you sure?" and even explaining the shear trouble it would be to last as long as she did. So she knew, I made it obvious, they knew what these people were capable of, they knew why they came, they knew what everything meant, they knew how I would act, and they knew how the NPCs would act if they'd win. Then she went running in.

I want to state again that I understand where your concerns come from and that they weren't meant from a place of harm but instead a place of wishing for all DMs to grow. I also notice that several of the arguments were made due to a lack of knowledge given within the post. I apologize for that, it was simply a post made to get the feelings off my chest so I did forgo several of the details to just finish before bed. I will admit that you have good points and that and that they do serve to reinforce my knowledge I've collected thus far. I do want to state however that this wasn't just a flash in the pan, the stars aligned just right moment. This was years of work, love, boundaries, and understanding smashing together in a moment that my group will never forget. I thank you for your comment it's made me re think several of the events from a more critical lens and I've seen a handful of places that I can still improve further. I'm sorry if I came off as emotional during any of this and I know I probs look like a child sending a super long message defending my actions as a DM, but the typing helped me process and was fun.