I haven’t taken any JLPT test, and not to diminish your accomplishment too much, but aren’t N5 and N4 rather negligible in the grand scheme of things? I have heard many times that N3 is generally the lowest acceptable, “You can speak some Japanese now,” level. You learn a lot of N4 kanji through the lower levels as well, and relearning basics is never a bad thing, and can only strengthen your foundations.
As for synonyms that should be included, I see you have been informed that you can add your own, so I don’t see the big issue. If you get something ‘wrong’ that you feel isn’t, just take a second, add the synonyms you think should be there and move on. Going back an SRS level isn’t the end of the world. Though trust me, I know what it feels like to have to wait an extra 8 hours to level up, lol, happened to me yesterday when I went to fast and typo’d.
Speaking of small typos, back when I was level 20 some years ago, I remember typing some things that were way off that were accepted as correct. I would much rather get something wrong and have to do it again than accidentally reinforce potentially wrong information in my head. The system is pretty lenient as long as the answer is over 4 letters.
Hopefully you can grow to like the system as it is, you stand to learn a lot here, and while the beginning seems disheartening (I reset 20 levels backwards and am retreading old comfortable ground myself) patience and understanding will help you so much throughout your journey here. Userscripts aren’t hard to use, though I would stay away from any scripts that touch lessons or reviews in any way unless you have tons of self control, they are too easy to abuse, and like some have said in this thread, the staff built the system this way to optimize learning.