Must Media Encoder be used to export to video?

It used to be that there were reasons to want to not involve AME. Without it you could get a large MOV that had an alpha channel and was using a lossless codec. If you used AME you might get a moderate quality MP4 file.

In the current version you have access to a long list of codecs, if you choose QuickTime as the format. Many of those are good enough, without taking up the amount of space that Animation codec took up. You can then take that file into Premiere, or just open it in QuickTime Player and export various resolution versions from there.

with the current version, you can stop auto-rendering but you can't stop it from opening ame.

with older versions you could and you can try the oldest version available to you.