I'd hoped she'd then be comfortable with it before we go away, but she doesn't really seem to need it,
From the sounds of it she doesn't need or want it. You mentioned less than a teaspoon of food and not really opening her mouth for it. If she needed/wanted solids she'd be showing more signs of willingness or eagerness IMO (mine screamed at me every time he saw food for an entire month before I finally gave in at 5.5 months as he had just gained the strength to sit, when offered food he grabbed and shoved it straight in his mouth, sure signs of readiness!).
her weight gain was tailing off a bit but I'd have thought that was a sign that she needs more milk rather than solids?
She'll get more calories from milk yes, milk is always the focus at this age anyway but if she has a small appetite then I'd say even more so the milk than the solids.
especially as we're staying with family for a couple of weeks and I know they wouldn't approve, but I know I shouldn't base our decision on that!
Oh hugs. It's very hard when there is pressure from others and you feel they are criticizing your parenting choices. A few things you might consider
1. practise now, out loud, what you will give as your stock response when asked/nagged about solids food introduction, eg "Oh yes I am so excited about starting solids with her, it's going to be so much fun finding out what her favourites are! Thing is she's just not ready yet, I did try her with a few bits but she wasn't interested" or "I'm really excited, etc, the most recent advice is the wait until they can sit up unsupported, she's not far off so I'm sure we'll be able to start soon" or whatever you feel comfortable with yk? I would try to throw in some sort of agreement with their enthusiasm so you aren't giving a direct contradiction on their parenting and your parenting. The thing is no parent likes to be criticized and many of the newer guides do tend to conflict with what previous generations have done (were told to do, did because the guides said to and they wanted to be the best parents), older generations don't want to be told they did it 'wrong' yk? I tend to find difficult subjects much easier to deal with if I script and practise the words many times before hand so I'm not caught on the hop without a response.
2. If you feel comfortable and confident you might offer her a piece of finger food. Prob on your knee, sat right up straight and supported. Your family may be happy to see her offered some food but she's unlikely to actually eat any based on her lack of interest.
3. Just hold off a few days or a week and reintroduce when you are with your family. They'll see she only takes a little and isn't all that interested so this may help you, but also they will love to see her having some first tastes too.
if we were to pause on solids, how long do you reckon it should take for things to go back to normal, if it really is related to solids? I mean if her BMs show no change after 3 days, say, would you suspect it's unrelated to solids, or do you think we'd need to wait longer than that?
If there was no change in 3 days I'd prob wait longer personally. Depending what her poos are like and how long it continues she might need checking over with a health professional - I know you are the best judge of that and you've clearly been investigating her BMs for some time etc.
One thing I've just thought of - is there any possibility she is constipated and the poo she is passing is getting around the blockage, so more frequent but smaller, looser poos, does she have diarrhoea just now with these frequent poos??