Suggest you ask the company providing subtitles to send you a file with 25fps Time Code rather than 50fps.

Open a case here: https://www.avid.com/learn-and-support/contact-broadcast-and-video-support

First is free. Ultimate hasn't been qualified to run on new iMacs with M1 chips. If you want to move a license from one machine to another you need to de-activate it, move it and then re-activate it. The first 2 items preclude moving the license.

Check the console to see if there are any error messages being thrown up during export. Or, do an export of the problematic section at the end. A few tries to figure out how far it goes before failing and then changing the marks and exporting again should help you locate the bad media. Chances are it's something added recently. Lift or mute likely

If you're using original source files linked AAFs work as long as the source file names are correct in Avid. If using Avid media AAF's with either linked or consolidated media works as well. Resizes and simple effects will carry over, color effects won't (duh). If any changes are made to the sequence you need to have Resolve make a new AAF

Markers will move if they're on the clips you're moving.

Evercast can work with just NDI or if you need to air gap your system you send the output from a i/o device on your Avid to a input device like a Blackmagic Mini Recorder or a AJA U-Tap on a second computer. It's optimized for real time, high quality, full frame rate streaming with audio staying in sync works very well, as it should for the price

Any different result with Avid media rather than linked media? Any different result when duplicating into a different TC column than Aux TC 2? Final bonus qustion: Try moving one clip at a time into a bin and see if it always fails or only after certain clips are in the bin.

Take a look at your new sources on your timeline and see if anything stands out, like 4K resolution or a different codec.

Nope. Avid will always send audio to the i/o device if there is one connected. You can feed the audio board with decoded audio from the monitor or using an external HDMI or SDI de-embeder.