First of all, the New Homes Ombudsman doesn't exist yet. I was promised/announced by the housing secretary, James Brokenshire on 1st October 2018. Currently an MP (since December 2020) Natalie Elphick is consulting (colluding more like) with stakeholders to draw up a Code of Practice which housebuilders can voluntary sign up to. This is way short of the statutory New Homes Ombudsman promised nearly three years ago. The "plastic" version may be operational around April 2021 but it will probably only be any use for owners with homes less than 2 years old. The New Homes Ombudsman when it finally exists, isn't going to adjudicate over warranty matters at all either!
As for the NHBC, you can complain to the Financial Ombudsman, but as it stands, the NHBC haven't said they won't be repairing the damage and the defects, just that you face a long wait for a decsion.
I would write to the NHBC CEO Steve Wood email
swood@nhbc.co.uk and suggest that any delay will result in the cost of repairing the water ingress damage will being even higher.
I would also highlight the disregard of Covid precautions and the "black hole for paperwork" comment.
Linden Homes (Vistry Group) are ultimately responsible not the window company they employed.
You should push to ask why, given their dire poor reputation, that Linden chose to use them in the first place!