Is it OK to do planks and yoga-style exercises like this: https://www.youtube.com/watch?v=4BOTvaRaDjI while deconditioning? I searched all over for an answer to this but forgive me if it's been asked before. Thanks!
Its not ideal, but probably Ok. You won't decondition certain muscles as effectively, if you do any kind of hard muscular contractions. Ideally you should avoid any exercise that uses your muscles for intense work. If you want to decondition, then avoid hardcore yoga moves or anything like that. Otherwise just don't bother deconditioning at all.
I find that static stretching during SD to be helpful. Just make sure you hold the stretch at least 30 seconds so the muscle does not think it is being worked instead of stretched. And no rocking to increase the stretch as that mimics contraction.
To be effective, your SD should be at least 9 days long. At 7 days there will be some muscle deloading but that is not long enough to be effective in breaking the RBE barrier.