I'm coming up with this response off the top of my head. Someone please correct me if I am in error.

If it's a Sunday, then I would believe the Sunday of Lent would take precedence over everything as I understand it. I'm not sure if it is even legal or licit to do so on a Sunday of Lent - it would most definitely put said damper on what is a momentous occasion in your parish's history. I would certainly presume that many if not all of the texts of the Sunday of Lent (orations, readings, etc.) would have to be taken from the Sunday of Lent.

If not, then I believe it is treated like a local Solemnity (like the anniversary of the church's dedication will/should be, the feast day of the patron saint, and others on your "particular calendar"), and hence organ and other instrumental music can be used ("Alleluia" still remains out, though). In fact, I believe the Alleluias in parenthesis you mention accounts for the possibility of the celebration taking place during Eastertide, whereby all antiphons in all liturgies have at least one alleluia added to them.

I hope this helps. Again, if someone notices I am in error on something I've said or can give better clarity to my off-hand response, please correct/add onto what I have said.