If nothing else, you did bring my attention to the fact that Nginx + Apache support isn’t included like I had been taking for granted.
I found this discussion here, for those who come across this post in the future, where Virtualmin staff voice the opinion that running both is unnecessary and will therefore never be supported.
I mainly wanted both for the flexibility, so I can use whichever works best for any given scenario. The performance and security benefits are just a bonus.
I get not being able to configure both simultaneously through Virtualmin, but why block my manual configurations too? That doesn’t make any sense.