My experiences as a ride leader.
I carry a couple of spare tubes, a basic toolkit and spare chain links to fit several speeds of chain. Most riders on my rides will be on 700c and i‘d happily lend them one of my tubes, but I would really expect them to carry their own. If someone snaps a chain, I’ll give them a spare link and with any mechanical breakdown or puncture, I’ll help them to fix the problem and get going again.
I am First Aid trained I don’t carry a First Aid kit, there’s only so much carrying capacity on a road bike and I don’t want to riding with a saddlebag/rack pack/panniers. We have had a few incidents over the years which required medical attention, but in all cases an ambulance was called. I think the idea of a space blanket is a good one. One if these incidents was in deep, mid-winter and it took the ambulance 40 minutes or so to arrive, so the injured rider was getting cold.
if your club/group is Cycling UK or British Cycling affiliated, then as a ride leader, you are covered by their insurance. However, in 40 years of doing club rides, I’ve never seen or been involved in any incidents which saw a claim being brought against the ride leader.