Dear Reader, it is better to have knitted and frogged, than to never have tinked at all!
Having gone back to basics and my original gauge, and with help from (thank you so much), I frogged back to the beginning of the short rows to correct lumpiness and errors. Changing sizes was therefore easy, from 3 to 4, as the pattern was identical at that stage. I dreaded doing it with two jumpers on the go; the second one had insurmountable loose tension problems from the chenille yarn. I felt that all my progress was lost and was feeling low.
To combat the gloom I reknit jumper 2 from scratch, and steam blocked after clue 1 was finished; with careful concentration, the problem was resolved and I felt better.
While that was drying, I reknit jumper 1 as size 4. It's now complete up to clue 3, and I'm really happy with it; the knitting lies flat without blocking, it's smooth and soft to the hand, I can't stop stroking it!
It was totally worth it to go back and fix. I've learned much about why I knit, what my expectations are, and how to approach my knitting more mindfully.
Now that I've got more stitch markers, I can catch up on jumper 2 before the release of clue 4. I'm happy only having one up to date, as the second is a present with no time constraints.