The /etc/xdg/user-dirs.conf denial is fixed in 2.29 (the core snap in the beta channel should have this fix).
The bluez denials are just noise and likely not the cause of your issues. Feel free to plug the bluez interface if you want them to go away.
The ptrace denial is almost certainly due to cascading failures (eg something earlier failed and mailspring is trying to trace it, etc). The ‘ptrace trace peer=unconfined’ is not allowed by any interfaces (it would be a huge security issue: you would be able to control any unconfined process (eg, systemd), dozens of processes on a classic system, etc). But, like I said, I doubt you need it.
The dconf denial is for mmap(). When I was looking at mailspring wrt interface auto-connections, I reported that the snap suffers from having an executable stack:
$ snap-review ./mailspring_42.snap
Warnings
--------
- functional-snap-v2:execstack
Found files with executable stack. This adds PROT_EXEC to mmap(2) during mediation which may cause security denials. Either adjust your program to not require an executable stack, strip it with 'execstack --clear-execstack ...' or remove the affected file from your snap. Affected files: usr/share/mailspring/mailspring
https://forum.snapcraft.io/t/snap-and-executable-stacks/1812
./mailspring_42.snap: FAIL
(snap-review is from the ‘review-tools’ snap in edge).
Please see Snap and executable stacks and adjust your snap accordingly.
The cause of your problems I think are the /run/user denials. Your snap is allowed to read things in /run/user/*/snap.mailspring/* that the user owns, but notice the denial has ‘fsuid=1000 ouid=0’. This indicates that your snap is creating /run/user/1000/snap.mailspring/.org.chromium.Chromium.7XwCRb/SS with root permissions and that your non-root user process is trying to access it. You mention this is an electron app: it sounds like you have the internal security sandbox turned on. Please specify --disable-sandbox or otherwise configure your snap to not use the internal sandbox and simply rely upon snapd’s sandbox. I am not an electron developer (but understand the issues surrounding having its sandbox turned on within a snap), but it looks like https://github.com/electron/electron/blob/master/docs/api/sandbox-option.md may be of some help.
In short, I think if you remove the executable stack and disable the internal sandbox, things should start to work for you (and optionally start using the beta core snap).