DS is autistic and I really wanted him to do the Bronze, because I thought it was a good introduction to volunteer work, and to get him thinking about how there are so many groups/ events which only happen because some people give their time to do them.
Thankfully I have a network of people whom DH and I used to volunteer with in various groups, I rarely see some of them anymore but they exist on my FB. I asked there and one said her scout group (something DS never got on with as a smaller child) regularly has volunteers for DofE in and would he like to join? Thankfully he did, and he's still doing it now a couple of years later.
Scouts/ brownies etc are ideal for that age because you're right, when they're 15 they're too young for many things.
I don't mean this unkindly but while I can see you're irritated with the DofE's 'requirements', it is meant to be something you're making some kind of effort to do. From all the suggestions here, I can see she's got a lot of reasons why she doesn't want/ can't do certain volunteer roles. She might need to consider Brownies given her age and the fact that she's already tried the libraries/ charity shops etc. She does have an option there and it's unfair to blame the DofE for her rejecting it, imo.
Anyway, in addition: the 'skill' element is quite vague and very easy, honestly, don't overthink it. DS takes a musical instrument which he does once a week after school. The teacher signed off on that as his skill. However, you can quite literally do any skill, cooking, whatever, and get someone to sign off on it. You could do something similar to what we did for DS's activity:
He played a LOT of Pokemon Go, still sort of does, and we do a lot of walking as a family because of that and well, just because we like walking. You can use Strava or just rely on a family friend willing to vouch for you that they witnessed him doing that. Yes it sounds a bit silly for it to be 'walking', but I saw 'geocaching' as an option on there so I knew it would be fine.
I overthought and worried about it a LOT until I learned, probably from MN, how many people were literally out there lying for their kids to get DofE gold. I was obviously not going to lie for DS, but it helped me realise there's a lot of flexibility out there.
This carries over to the volunteering a bit: many (most?) people would be happy to sign off on a chunk of volunteer work at a time, if she truly can't find something that is exactly an hour a week. If she does 3 hours for someone doing whatever, see if that person will sign it off as 1hr/ week for 3 weeks. Just as an example. I know someone whose DS did this for their school's PTA; he basically volunteered to work at most of their events/ did some kind of graphic design work/ plus some other thing. He actually did way more volunteering than the 1hr/ week in the end.