Yes to all three questions. Very much a joint affair. We've always been that way and I wouldn't feel a need to put it in writing. Maybe I'm too much of a romantic? I'd say marriage is on the cards within the next few years anyway.
If there are tax implications for making overpayments in this scenario, it'd be a difficult to forget them as they might defeat the purpose of making overpayments in the first instance.
We'll probably make equal amounts of overpayments. I suppose what I'm wondering is if a windfall of €10,000 comes our way, can we stick it into overpaying the mortgage without having to worry about tax liabilities arising for the other person?