Yeah I'd agree with Bani that the proportions are off Red. Think partially it's the neckline of the shirt as that's dropping another horizontal line in and framing you down and the bottom volume squishes you down further. I can't do red and black together after a very long uni lecture about who utilises that colour combo and why (TLDR; extremists, totalitarians, etc. and they're colours that naturally trigger us and put you on alert). It is a tough combo to wear without losing yourself a bit.
I suppose it's quite punk to mend with some glue you happen to have around rather than doing the proper traditional mend, Bani. It's why I find the Rive Gauche punks getting their hair and nails done so hilarious, it's so counter in all the wrong ways to be counter by punk standards. I think they're definitely missing a trick by not having a little customisation pop up stand too, where an expert tailor will rip apart your newly purchased clothes and carefully suture it all back together with safety pins, coarse stitches, maybe some tie grips and duck tape, and flick a bit of fabric paint about. Although the poor tailor will probably be in sobs at the end of the day.
My first instinct would be vinegar, potentially with some bicarb, quirky. Though IME set sweat stains are hard to get rid off without bleach.
That jumper looks much nicer on you than on the model lightning Colour looks lovely against your skin too. Knitwear tends to sag on me, so I'd go for something rather on the shorter side than long as it'll get there over time anyhow, but you won't be able to afford an accidental mishap by washing it too warm.
I like Hanro's v neck camis, here though I'm sure other UK retailers available, I know Matches sell them. They come pretty low on me, just about cover what a bra would. Although these are the new seamless ones with adjustable straps so I can't vouch for them completely. I've had mine for forever, the oldest one is over 15yrs old, the tag is completely illegible but the rest of it still looks as new. Sizing is a bit odd, they are fitted to the body but an XS is about a size 10. I have a few in an S and they fit me fine