So I think you need to look at this for the view point of having rented it to an independent third party for 6 years and see where you would be with the mortgage and then make a call on what "compensation" would be reasonable.
This is absolutely the first step. I could not follow your description of who lived where. At one stage, it appears that there were 3 couples living in the house, but one paying a lot more than the others. At other stages, there were three of you living in 4 houses. I will assume that the mortgage payments since you bought it reflect the use and no adjustment needs to be made for this.
From an investor's point of view, it is pretty much irrelevant what they paid for it. It is irrelevant what they spent on it since. It's irrelevant what income they got from it. (Leaving aside some small tax considerations.)
Today, they have a property worth €290k and a mortgage worth €290k.
If this mortgage were at normal market rates, then they could sell their share of the house to you and no adjustment would be needed.
If I owned a house worth €290k which was generating €20k rent on a tracker costing me €3k a year, I would not want to sell it. That is €17k profit a year which is a very nice investment. If I owned a third of such an investment, I would probably want around €20k to give it up. In other words, I would sell my share in the house for about €120k.
If you ended up with a house worth €290k and a tracker mortgage of €290k and you had run down your savings by €40k, I think you would be doing well.
That is very much an off the top of my head figure. I would do a longer calculation later depending on the following.
The bank is under no obligation to allow your wife take over the tracker, however, they may allow you take over the mortgage if the income of yourself and your wife is enough to meet the repayments. They may well ask that some capital be paid off the mortgage to reduce their NE exposure.
If the bank does not allow this, then there is no point in you taking over the mortgage. You would have to compensate them for giving up their valuable investment, while at the same time you would be destroying the value in the investment by losing the tracker.
So the first step is to approach the lender to ask if they would allow it. If they don't allow it, then you don't proceed any further. Everyone stays as they are.
If they allow it, then, you would have to do a complex calculation. There is no point in doing that now, until you hear from the bank.
Brendan