Salisbury and Bath (advice on Stonehenge pls)

If you are only mildly interested in Stonehenge I would skip it. It will certainly complicate your agenda and leave you less time for Bath and Salisbury. The latter two can be done in one day, provided you are satisfied with seeing the highlights, as opposed to an in depth visit. Many years ago we did Bath, Glastonbury, Stonehenge and Salisbury in one day. We took the train to Bath, then rented a car there for the rest of the trip. It was a long day and I desperately wanted to spend more time in Bath, but we loved it. Fortunately, we've made it back to Bath two more times.

At Bath, be sure to see the Roman Baths and Abbey. Then walk to the Circus and Crescent to see the beautiful Georgian architecture. The whole city is quite beautiful. I haven't seen any art or other churches there, so can't offer anything else. In Salisbury, of course you will want to see the cathedral. That is all we had time for on our quick visit.

Sorry I can't help you with the trains.

The cities are as safe as anywhere else in the UK.

You can use the trains as you are suggesting, as long as your return train stops at Salisbury. Personally, I would scrap Stonehenge - you can't get right up to the stones and it is expensive to get in. It is 30 minutes each way by bus from Salisbury so I don't think you have time for this and Bath. Salisbury Cathedral is worth a look if you are stopping there.

In Bath, try a free walking tour (2 hours), then there is the Baths and the Abbey. Most of the city centre churches should be open during the day. There are plenty of independent shops in Bath including lots of galleries.

Sienna, anywhere in the tourist areas in Bath you will feel safe. There's a vibrant university community in Bath too, so students are a big part of the scene. The (free!) walking tours in Bath are well worth it. The Abbey there is stunning, the Roman Baths (and related museum) fascinating, the town itself charming. I've not been to Salisbury, but I can tell you that I could easily fill a full day in the small area around the Bath train station (much less other areas of Bath) by myself. I'll leave it to others to comment on pure logistics, but to me combining Bath with Salisbury will leave you wanting more of one, the other or both. If it were me, I'd vote for Bath in a day visit at a more leisurely pace. As for Stonehenge, I'd definitely skip it. It is not close enough to make it worth your while on this trip, includes mass tourist groups seemingly at all times (robbing it of its charm), and a price that oustrips its value, in my opinion.