Can you give an example of what sort of entries in
/proc/self/mountinfo from within a strictly confined snap are producing incorrect information from df, etc. and what those entries look like outside confinement?
Unfortunately, even with classic confinement we cannot guarantee that these mounts won’t appear some day with a new snapd release because features we want to add to classic snaps (such as the experimental parallel snap instances or having content interfaces work with classic snaps) require entering into a new mount namespace, so if your application really cannot operate with the additional mounts inside the mount namespace then it may not be a good fit for snap packaging. You can test this yourself by making your snap classically confined, then enabling parallel instances and trying to install another instance of the snap.
We could some day introduce some form of proxying such that when an application tries to read /proc/self/mountinfo from within the sandbox and the snap has a special interface connected, snapd will intercept this read and replace the real output with whatever the host’s /proc/self/mountinfo is, but that would take a fair bit of work we don’t have roadmapped currently.