To answer you questions:
1 The fighter and paladins were GWM and very much optimised. The party was having a cleric and a bard in addition to the other four (yep a party of 6 player characters. Just like I am having at the moment with two different groups).
2) The normal magic missile can't be twined. But we had a version that could only target one target at the advantage of a higher range and the ability to target objects. I should've mentioned that. Spell research was and is still a thing in my games. Any single target spell can be twined so... The problem stays in its entirety. Some campaign lasts 40 years (of in game time, not IRL obviously) so the players have the rooms to create new magic. Sometimes a spell does a good job and is kept from one campaign to the other. At other time, a spell is completely dropped off because of unforseen side effects. (Such as the magic lance, the modified magic missile).
3) I apply monster variety in almost every encounter. Some monsters use the bloc a corridor and dodge tactic while their allies blasts the players in the back row all the while concentrating on haste so that the front brute can use their additional action to dodge. My games can be brutal with effectiveness and I do not hesitate to TPK. The enhanced spell damage was so good that we were only seeing the enhanced spells being cast and even at high level. Especially if the enemies' AC were high. When a tactic is so good that no other possibility is considered whatever the circumstances, you know something is wrong.
4) The campaign ended at level 19th but we ended boosting spell modification at around level 18 where the last power boost to cantrip occured. (I know it's 17th but we gave it a go for a full level to check if we were not mistaken). The modified magic missile spell was dropped down from all campaigns after that one as the twining of MM was deemed a bit too efficient in the hands of a sorcerer(ess).
As I see it, the damage bracket in which arcane spell casters are at the moment is the best that they had in decades. Against numerous opponents, they shine with mass effects like fireballs, meteor swarms and the like. Or they control the battle with spells like walls, clouds or whatever comes to mind. At the same time, against single targets they are left a wee bit wanting. Which is quite canonical in litterature where the "evil" wizard blasts the hero who shrugs the damage (with luck or skill) and goes to kill the mage in hand to hand combat (or a well placed arrow). The days of 1st edition are long gone. The philosophy behind classes is one of balance or at least attempted balance over discrepancy in power at all levels.
I hope I have answer your questions.