Of course Meghan can wear a tiara the etiquette and I'm sure some one will correct me if I'm wrong is that tiaras are worn by women on the wedding day or married women attending appropriate black tie evening events.
So in theory Zara, Autumn and Eugenie could all wear tiaras. In reality they don't tend to go to tiara worth events. So yes Charles coronation we will almost definitely see Meghan in a tiara who knows before that though.
I think It will depend a lot on the real question which is what is a senior royal? Will the still be HRH's are they still the commonwealth your ambassadors, I'm guessing they won't do formal a royal tours but that's a guess.
For me the only thing the statement seems to say for definite is that they are going to fund themselves (eventually) so they can do what they want. How their role will look going forward I'm not sure any of them know.
I hope they can find away forward that if good for them as I agree with the pp the pointed out they have both been very proactive and positive for the charities they have supported and while I'm actually not pro royal (I just like gowns and bling) I don't see what they have ever done that is worse than any of the others.