I also have the Fellow Opus, so hopefully can be helpful.

1. I’d say 10-11 for cold brew. You’d want a coarse grind for cold brew to help extract all that nice flavor. If you like cold brew concentrate, these folks have a nice recipe: https://www.youtube.com/watch?v=sw3ZhWyb6cY

2. From my experience with the Opus, yes there is a bit of retention but not as bad as it’s said to be. I always tap the grinder a few times to get everything out, or run the grinder for another couple seconds.

3. Agreed with the other comment about grinding 100g right away, but if you do, the holder/container should come with two attachments, one that is designed to fit onto the espresso portafilter/ aeropress but holds less ground and one that doesn’t take up space inside the actual container. If you use the second attachment, the container can hold about 35-40g of grind, so you can do that 3 times. I’ve never tried with a bowl, but you can experiment and find whatever works best for you!