Oh boy ...
Arnottʻs are known for being the worst guides on the island. They are certainly not suitable for a person with medical issues. These are the guys who have left customers in distress, alone on the lava fields, if they canʻt keep up.
If you do want us to help you, it would be helpful if you come back with an idea BEFORE you change to it.
Again, you cannot just go "hike" at Kalapana with buddies. You have to go with a guide, and the cost is per person, so the only cost to share is the transport.
Also, while this island is a lovely safe place for 99% of the visitors, the people who end up going missing and never heard of again are the people who hook up with strangers while on island. There has been a pretty consistent history of women disappearing, in particular. There are predators watching for women who are all excited about being on vacation and who are being open to stuff.
Youʻll do what you want, but Iʻm not going to say itʻs a good idea. Even couples and single guys can get into trouble. For a single woman it is just not smart, safe travel. Stick to tours if you canʻt drive, and not Arnottʻs tours if you have medical issues.
There is a hostel in downtown Hilo where you can walk to everything for food, farmerʻs market, etc.. Arnottʻs you cannot walk to anything like that.
You might look into Vavoom tours. I donʻt think they are as expensive as Volcano Discovery, and they do pick people up. They have a good rep.