I would see if the metacam makes any difference and then see what the other options are and have a good discussion with a rabbit-savvy vet.
Metacam can be given on food that will absorb it - so pellets, a shreddie, a small crouton-sized piece of bread, etc. Most rabbits quite like it and will take it willing from a syringe once they get used to it.
Insurance may not pay out for dentals and associated X-rays without a good reason, so I would double check that it would be covered by the policy, or be prepared to pay it yourself. If shes having an anaesthetic anyway, it might be worth asking for x-rays of the spine / hips if suspected mobility issues, and the skull to check on tooth roots, if none have been done previously. Talk to your vet about whether it would provide anything useful which could help with future treatment.
I don't think there's an age limit to arthritis in rabbits. Some seem quite old at 5 years, some of it is breeding / genetics, and some of it is their living conditions. If she's had a history of being confined and not been able to exercise properly, it could have an impact on mobility which will get more noticeable as she gets older.
The grass advice does seem odd. Usually the advice is to cut out other processed food and increase grass / hay consumption, not reduce it - that's what keeps the teeth worn down and the guts healthy. It is their natural diet, after all. It's good that she's maintaining her weight.