The reason for why the backend for the snap store hasn't been opensourced has been explained multiple times. Namely that it would be expensive to open source it with little benefit in return. Canonical already spent a large amount of investment opensourcing launchpad and nobody other than them operate it. Mainly because the majority of the costs are for operating an instance which most other distros aren't willing to…
Would you describe exactly what will be expensive in releasing the source code to software that was developed in house? Does it have lots of dependencies on proprietary software? If so, why?
> Namely they want one location to find software, and one location to serve software.
And this, is the killer. This is exactly what the people at Linux Mint do not want. This is why F-Droid exists on Android. Yes, free software can be distributed on the snap store or on Google Play, but ultimately these are proprietary platforms controlled by one corporate entity whose interests may not be aligned with those of the community. Users of community Linux distros demand choice in where they obtain software.
> That and the whole aspects of malware/trust goes out the window.
Malware has already been successfully pushed to the Snap Store. The idea of "trust" here is also antithetical to open-source and software freedom. Rather than trusting software because it has verifiable source code, we now trust software because it came from a store that removes well known malware.