Welcome to the Forum, SaiTalos!
I don't have answers to all your questions, but here's what I can tell you:
1. I'm pretty sure that you'll go back to level 10 if that's what you were previously. There's also a cheat to place your Sim at any job level if that doesn't happen.
2. I haven't tried doing that. I think there's a serum that your Sim can make at level 10 of the Science career that will make your Sim a YA again. You might be able to do it with a CAS cheat. You can use the Immortal potion any number of times, but it will only take your Sim to the beginning of the current age. You can also turn aging off in the Game options or set it to long -- 4 times the normal rate of aging.
3. Onions can be found in Oasis Springs. Only a few plants are available through the seed packet. You get the rest of the plants (seeds) by going around and harvesting things in the wild. You'll usually need to send your Sim to an area to get plants to start growing though, and visit several times if your Sim doesn't live in the neighborhood.
4. I don't know about this -- have you tried making your Sim do the cleaning?
5. The bathroom glitch is a bug. If you move furniture around, your Sim will be able to get to the toilet again. I've not had the second bug, but it sounds like a glitch. You could try repairing your game (right-click on the game before you open it, and "repair" will show up), or try reinstalling.
Carl's Guide has plenty of useful information, too. If you're interested in gardening, I'd definitely recommend the gardening section.