I spent a lot of time bouncing around the Balkans in 2015 as a solo, 63-year-old, female traveler. It was my third trip to Croatia/Yugoslavia, and it was much easier in 2015 than in the 1970s/1980s. A lot of people in the tourist sector, as well as younger people in general, speak English, though that's not likely to be the case with older bus drivers. It may occasionally feel just a bit adventurous, but I don't think you'll have significant challenges.
I found the buses generally reliable, but I was very diligent about getting schedule information. There's more available online now, I think, but even so, I always checked at the bus station. On occasion I stopped at the local tourist office to ask about buses, and I noticed that for long-distance buses they picked up the phone and called the departure bus station rather than turning to the internet. I figured that meant they were a bit uncertain about the reliability of the online info.
If your tour doesn't cover it, I'd super-highly recommend a visit to Plitvice Lakes National Park. You'd need to spend the night in or near the park, though, to avoid being caught in the scrum of daytrippers. It's super-important to be positioned to enter the park when it opens in the morning, though I've read here that conditions are not usually not too crowded really late in the afternoon, either. I think the park sells a two-day ticket, and in the past the hotels (rather expensive) inside the park would validate a one-day ticket for use the next morning. You might find some less expensive options right outside the park on booking.com. Perhaps some of them are willing to shuttle visitors to the park.
The one bus challenge I faced was at Plitvice; I wasn't sure where to wait for the bus to Zagreb. In the end I found a bus shelter that even had a schedule posted. However, the bus I took to Zagreb arrived at an unexpected time, indicating the schedule information available in Zagreb was at best incomplete. It definitely wouldn't be prudent to plan on taking the last bus of the day in that sort of situation.
As of 2015 there was a modest charge (roughly the equivalent of $1) to place luggage in the hold of the bus. If you have a small carry-on bag, that may not be an issue for you.
Bus tickets are likely to have seat numbers on them. They are often ignored by both passengers and drivers, but not always. One driver insisted that we play musical chairs so everyone's location matched his ticket.
My lodging choices were made not long before arrival, based on price. They ended up being not typical of my usual budget hotels. On my first pass through Zagreb I got a really good deal (it was probably over a weekend) at a Best Western between the train and bus stations and the historic center. It was air conditioned (essential) and quite comfortable. It was a bit of a walk from the oldest part of town but close to some of the large museums, so it worked find for me. My second stay in Zagreb and my overnight stay in Zadar were both in Airbnb-type locations, though I found them on booking.com. Croatia has a long history of private room (soba) rentals, though perhaps less in Zagreb than in smaller, touristy places. If you run into a problem and are in a bind, go to a local travel agency. They usually have a book of private rooms for rent. Be aware, though, that if you're in someone's home, you are not guaranteed of a smoke-free environment.
I basically ate where I was when I got hungry, so I have no particular recommendations on that score.
In Zagreb I recommend both the Museum of Broken Relationships and the small but excellent naive art museum. As of 2015 they were both located in the historic center--I believe on the upper level. Zagreb has many, many other museums. The tourist office, which at least used to have a branch upstairs at the very busy bus station, was able to give me an English-language pamphlet describing all the museums.
