Just off the top of my head, the fighter is linear and arguably the strongest class in the game. But it is helpful to have high level casters because their power grows exponentially. The trouble is, it's almost impossible to get a high level single classed M-U in RAW AD&D unless your DM is deliberately using kid gloves or you cheat.. So you right off the bat solve my problem. I go 90% fighter and 10% or so M-U until I get sufficiently high level in fighter that I've upped my hit points (6th sounds like a good break even point, though 10th is probably optimum in the long run so as to get 100+ hp M-Us). Then, I switch from there to 90% in M-U and 10% in fighter. Now I quickly level up M-U, and I'm tanky enough that I can actually survive with decent enough THAC0 that throwing weapons from the back line actually contributes. I power level through the low XP requirements of gaining levels as an M-U now with all of the benefits of dual classing and none of the drawbacks.
So you end up as a F6/MU6 once the MU side catches up, meanwhile all the single-classers are 7th or 8th. Same difference as if you'd gone with a 50-50 split all the way along.
At which point she's now perfectly positioned to switch from 90% fighter to 90% M-U to "please her family" or whatever RP excuse she wants to give. That's near spot on the power gaming curve. And really you can see how you are being scammed even your example. Had she gone 100% fighter she MIGHT have gotten to F-9. But by giving up 10% of her XP she's gaining net 4 levels of class on you. Now granted, this isn't the best way to make a OP fighter (that would be to dip 10% cleric), but it's heck busted if you are planning long term M-U.
That would require some
serious long-range planning, given that to get to F-8/MU8 (via any sequence) would in our games take potentially 6-9 years of play if not more.
The most common multiclass combinations I see are Fighter-Thief and maybe Fighter-MU, though lots of different ones have been tried over time. Long ago there was a time when various people wanted to go Ranger-MU, until they came to realize just how long it took to bump given that those are two of the slowest-advancing classes.
Literally the only thing that might stop some power gamers from abusing the heck out of this is you seem to be using the Dragon magazine rule that only single classed fighters can specialize, because specialized single class fighters have almost no draw backs until 13th level or so - which your campaigns don't seem to hit.
Only single-classed Fighters can specialize, multi-classed casters don't get as many spell slots as their single-class counterparts, multi-classed Rangers lose some benefits. About the only classes that don't suffer much through multi-classing are Thief and Assassin, intentionally so in order to make them a bit more appealing.
And you roll an average of the two classes' hit die, based on the proportion most often assigned through the previous level, when the leading class bumps. Admittedly, someone could game this a bit by running as a 90-10 F-MU and rolling d9 each level, then flipping to 90 M, 10 F and not rolling any new hit dice until the MU side caught up (unless the F side happened to bump at some point from that 10%, at which point you'd roll a d5 for new hit points because you're mostly MU now) but rolling d9 rather than d7 gives on average maybe 1 extra hit point per level. In the long run, who cares?
Sure, I mean that's not a problem. You just don't play an elf, which was already a really weak choice to begin with RAW. You've done away with all the reasons to do so, since you can multiclass as a human and nerfed probably the strongest benefit of being an elf. No reason to put up with that nasty -1 CON if you can get better AC than usual.
By RAW, Elves rock. They get proficiency with longsword and bow as a bonus, they can cast in armour (which nobody else can), they can see in the dark better than anyone else, and their (Int or Dex, I forget which) gets bumped up. Not bad just for losing a Con point, given that their other big drawback - they can't come back from the dead - might as well not have existed once it got out in the wild.
And how am I getting better AC than usual in comparison with any other Fighter?