Under IPC, you can run an unlimited distance between the toilet and it's vent, no more 10' limitations. Also under IPC, that vent coming off of the toilet running flat is not allowed. Minimum vent size is 1/2 of the largest drain served, which in your case could be as small as a 3" drain, so 1 1'2" vent. Yes, you can revent the lower fixture vent into the bathroom vent in the attic, and yes, you can revent that vent into the main vent for the house.
Lake County, Florida plumber
Springtime Plumbing Services