Hello,
From Track request: 1.9 for modem-manager snap the intended use was 1.9 was to “allow us to publish a new version of the modem-manager snap meant for use on Ubuntu Core 18”. Per what you say, the 1.9 software was never used for really stable builds - it seems to me as if a track was not required in this case, perhaps the builds using 1.9 could have used “latest” or a branch instead? We really prefer tracks to contain stable versions of the software that you intend to keep maintaining even after a new version comes in.
So:
1- +1 from me as a reviewer since I know you understand the use case for tracks and will try not to have this happen again (fast-track process since you have existing use of tracks). The track is now available! Enjoy.
2- Tracks can NOT be deleted 
What you can do is just snapcraft close modem-manager 1.9/beta (ditto for edge) and that will render the track pretty much hidden/invisible to anyone else.
- Daniel