I hope I don't offend here but this situation impacts you as much as it affects your partner. Why is it reasonable that you are not allowed to be a part of the discussions as to why this must be done and how bad their financial situation is. If my husband told me that I would be financing his parents retirement without an opportunity for me to openly consider all options and discuss with them I'd be livid. Honestly it would've even be an option for me so I guess I'm finding it difficult to understand your position and how you are so fine with it.
I would not be offended by a legitimate opinion so don't worry! If the situation stopped at we sell the blocks, buy their home, and that is the end of it, fair enough, it doesn't have much impact, so not overly concerning. Beyond that, if more is needed then I want to be prepared.
His parents are very private people and would be embarrassed I expect, to have me witness their need. It would have been very hard for them to admit to their son that they need help so why would I want to further that pain for them, I would not want to add to their discomfort.
If my husband went ahead and completed the transaction without my knowledge then yes, livid would be the tip of the iceberg. But he hasn't, he has been upfront and has said he would like to do this. When he knows more, he will also discuss this with me but ultimately, I know he wants to do it. He is 'allowed' to have a say, as am I, for the sake of pride, my say will happen in private and then a decision will be made which will probably be what his preference is.
I guess everyone has their trigger points/limits for each situation, this doesn't affect my 'limit' in the same way you mention it would for you. I only want to find out how to do it with the least impact, financially and without causing people I care about unnecessary embarrassment.
