Congratulations, you managed to summarize Janna's kit. Unfortunately you forgot to add an argument as to why you think it's overloaded.
-Excellent engage and disengage with her ult and knockup
Her Q and ult aren't "excellent" engages, in fact they're generally defensive CC. "Excellent" engages are something akin to what Leona can do.
-Free AD for allies
If by "free" you mean AD gained from a shield that -once removed- loses the AD bonus, and of which also gains a lot of its AD from AP scaling, then yes, I agree.
-Free movement speed for allies
It takes up her passive slot, and that's all her passive does. A movement speed aura. It's about as "free" as every other passive.
-A shield that works with turrets and allies
Her being able to shield turrets is interesting and unique. No other champion can do it in the game, nor is it overpowered. The fact that you think there's something wrong with that doesn't make sense to me.
-A heal with her ult
Which is only really effective with AP.
-She has a slow
^Describes half the champs in League.
-Ignores unit collision
That she loses upon using Zephyr.
Maybe there are other things you could criticize Janna for, but her kit being overloaded isn't one of them. The most "overloaded" thing she has is Zephyr, which isn't exactly a mindblowing ability in what it does.