1. When you merge 2 shipping companies, does the new company have a capacity of the combined values of the previous companies
If by capacity, you mean number of ships permitted by the company in each era, then yes.
2. Must a shipping company expand adjacent to a current zone it occupies? Or, can they be placed anywhere?
You can place it in adjacent sea zone, or place another ship in a sea zone it currently occupies. Not anywhere, no.
3. I saw a response to a question that suggested that a company cannot expand outside of its own province. Is that accurate? If so, why all the arrows connecting different provinces, if not for expansion?
You can expand a company outside of its own province provided:
1. The production zone you are expanding to is adjacent to the one you're expanding from. For island production zones, look for the arrows to determine adjacencies.
2. There is not a company of the same type in any adjacent production zone to the zone you are expanding to.