I have the Safariland holster mounted directly to the Invictus holster hanger. In that configuration, it does not correct the forward cant. It can also be mounted to the QLS mechanism as you describe.
I am not a fan of forward canted holsters either, so I feel your pain. I purchased the Invictus holster hanger to use for training only, so I also don't have any concerns with how strong it is in regards to gun grabs.
I don't have any experience with the QLS mechanism.
It appears that the female side of the QLS mounts in straight-drop fashion to the Invictus holster hanger. Having said that, it depends how the male side of the QLS mounts to the Safariland holster itself as to whether it fixes the cant issue.
Might be worth it to reach out to Invictus Practical for an answer to your question.
For what it's worth, I dremeled out the holes on my Safariland UBL in a horizontal fashion so that the holster now mounts in a true straight-drop fashion. Lock-tite and the right amount of torque has kept it secure. Has worked well for me for a few years now.