If I opt out will I not be able older mods that require older versions?
If you opt out of betas you will receive the latest, current version (1.10.1 as of this writing) of HOI 4. You won't be able to play mods that require older versions.
I'd rather not do that, I don't want to return the DLC to steam, if I opt out, can I at least opt back in?
Yes, you can switch back and forth between the current version (opted out of betas) and older versions (opted into) whenever you want and as many times as you want. Each time you select a different version, Steam needs to download files to your computer to bring the game to the requested version. So if you have slow internet connection or data usage caps, you may not want to switch versions too often.
Follow the instructions given
here. It's the same procedure whether you opt in to an older version or opt out to the latest version, you simply choose a different version in the beta menu (Step #3).
Paradox provides their own instructions
here, if you want to refer to them, but I think they're too complicated and confusing for the average user.
Can I play the DLC without opting at all?
The Battle of the Bosporus DLC, I assume? No. This DLC is based on, and requires, the latest patch 1.10.1 or later.