Magic Items "Waking up" or "Unlocking"
a perfect way to do this would be to supply them with powerful magical objects
So, one of your stated goals was to give them magic items from an already passed encounter. One way to do this without rewriting the events that have already happened, is to have a item they got from one of those encounters "wake up" -- in a critical moment, when the player does something epic, or in respnse to a new type of threat. For example, say "that normal sword you never use but always keep on your belt, suddenly it starts to feel warmer. You notice a faint glow of light coming from the inside the opening of the sheath..."
My current game's Mcguffin are already planned to get more magical when they are "unlocked". You can also see this at work with important items on the Geek and Sundry show Critical Role. In both of these cases the item is already magic, and they become stronger/more important -- but the concept could be carried over to currently mundane items being only "seemingly" mundane.
Hindsight or Dreams
From experience, is there a strong method of introducing new material into existing events without retconning a session?
Likewise, to make something in the past more important without completely changing the story, you can have a party member(s) experience a dream about that moment. In that dream, they realize something that was always there that they missed at the time. This typically feels more natural than simply saying, "You guys remember when X happened? Well, Y happened also."
Since we experience hindsight ourselves, it isn't hard to imagine our characters having similar hindsight. Now, this can really only work for subtle changes. The more changes made in this way the less realistic the injection seems.
(DM narration) As you sleep the previous days battle looms in your head, and you realize that there was a huge frost giant lurking in the background
is way less viable than
(DM narration)As you sleep, your mind plays over the events of yesterdays battle, and begin to focus on the details that were to quick to notice consciously in combat. Now, in the quiet moment of slumber you realize the bandits all had a matching tattoo on their left hand. The tattoo was in a language you can't read, but somehow, you still get the impression of extreme evil from the mark.
I occasionally use dreams with Warlocks (messages from their patron), Paladins and Clerics (messages from their gods), and/or Divination Wizards (it makes sense that they might occasionally see something that was/is/may be because of their close association with divination).
What the Players Don't Know
One option I have considered is using some of the predatory creatures living in the tower stalk the party and lead them into a hidden lair, but this adds significant time both in and out of character.
What the players don't know can help you. They need to leave these ruins through the way they came in. They likely looked for traps, making perception rolls, but they don't know if those rolls were good enough to catch everything. They could have missed something -- especially in an archwizard's tower. You could easily have some members (maybe more fun if it isn't all of them -- if you can switch between the groups well -- because then you have party member on one side trying to figure out how/where the others went and the others trying to figure out how to get out of this wizards private store room) accidentally set off a glyph spell that holds a teleportation spell to the secret chamber.
It could even add an extra puzzle if you want it to. The overhead wouldn't simply be tracking a beast but instead a puzzle with the extra pressure of needing to get back before that terror they felt gets to the tower and kills their friends.
The "what the players" don't know concepts can apply to other things, too. Maybe a note or book they already have contains a secret message. Or maybe an item they have has writing on it in a place the players wouldn't have thought to look. Maybe a trinket they took/stole on whim has special importance in a ritual even if it is mundane item. Maybe someone really wants that teapot (they threw into the bag holding to sell later without a second thought) back because it is a family heirloom. The details weren't in your original plans, but since they weren't important to the party at the time, they didn't care about/know about details. Those details can be anything you need them to be, without having to rewrite the parts of events the players know about.
A good example I've seen of this getting a bit out of hand was in Heroes of Awesome: A War in Scarlet's first simple quest. The DM accidentally made the first McGuffin seem more important than he meant. The party didn't want to give it to the quest giver, because they were led to believe there was more to it than there was. The DM chose to roll with it, and make it more than it was. When this led to the party getting stuck trying to figure out what to do with it instead, the DM wrote it out of the story in way that opens up even more questions about it.
If you do this right, the players will think it was intention from the beginning.