I do not own a 7d II, so I can't comment on your issue
How much over exposure do you get, is it a 1ev or more?
Although it does not answer the question in your post, my five cents on using the MT-24EX:
1) the MT-24EX is dated, I can not make changes on the flash via the camera as i can with the 600s
2) in E-TTL mode it works fine on my 1DX, at least to get right exposure, as long as flashheads are aligned with subject, see 5)
3) I use it for close up/macro shots, so I immediately switch to manual flash control on MT-24EX
4) the user interface of the MT-24EX is not friendly (I am in a diplomatic mood), so it can be difficult to get it to do what you want
5) please be careful that the flashheads are PERFECTLY aligned with subject, "one click" on the heads makes a very big difference when working up close.
Let's not forget that the MT-EX24 has not been updated for a long time. It is a tool for the specialist and as such it works!
Hope this helps ( a little bit)