1) I don't think so. It's one thing if someone's working from home in lieu of going into the office, but entirely another if there are clients visiting.
2) Messy. She should be responsible for it but may not have the appropriate standing with an insurer where there's already house insurance in place. If you actually wanted to go down this road (and I wouldn't!), the best thing would probably be for you to check your policy with your insurer and find out how much extra it would cost, put in in place and then recoup from her. She could then declare the cost as a business overhead for tax purposes.
3. It changes the character of the tenancy. You'd need to amend the lease and get good legal advice on any implications for you. Off the top of my non-expert head, apart from the possible insurance liability there's a planning implication (almost certainly need to get consent to "change of use") and probably liability for commercial rates.
If it were my tenant, I'd say no - there are just too many problems.