I did in Russian. It was years between my first attempt and my second and I didn't really get far the first time so I decided it was better just to start from scratch. If you got quite far though I would imagine it is not worth it. It's really up to you though and I doubt it really matters.

I wouldn't, long term it really doesn't matter. Just keep reading you'll eventually just re learn the words. Don't get too caught up on the known words count (I'm totally guilty of this).

Just read read read!

For me it would depend on what level I was before I stopped looking at a language regularly. B1 or B2 then probably not. A0/A1 definitely would consider it.

If I still felt somewhat confident in the language (High A1/A2) then I may consider instead just going through old lessons and changing the 1-4 number to match my current level of understanding for each word.