Can legal guardian make funeral arrangements for Mom when family and spouse refuse to participate after written request sent to them?

As OldSailor says, guardianship is while the person is alive, as are POA and probably conservatorship. The executor named in the will takes over after death. Check with the executor if at all possible. Another letter to the 'family' explaining what daughter proposes might be a good idea. However it might just stir up trouble, as some people don't like the idea of body donation. Perhaps another repeat request would be less disruptive, just explaining that mother is on hospice and the end is nearer. You may think that saying there will be a memorial service or some type of get-together will be a good idea, and perhaps ask for suggestions.

If there are no replies again, it is probably reasonable to assume that no-one cares and no-one will try to intervene or object. If daughters can show that all reasonable steps have been taken to ask, there is unlikely to be any substance to an objection after the event. There are no monetary damages to allege. Perhaps ask a lawyer if you think serious objections are possible (for example, some religions believe that the body must be buried intact to allow resurrection). After another letter and an executor check, I agree - just go ahead and make plans.

Guardianship usually expires when the protected person dies. I would say to make plans now and not wait. Maybe a letter to the rest of the relatives and see what happens.
But a certified elder care lawyer in your area should be consulted just to keep things straight.