Agree with those who say that this could have been sorted out with an apology for the wet floor and your or your children cleaning the mess up without resort to the excuse of mental health issues. I can imagine being your MIL, miffed at nearly slipping over on a wet bathroom floor or scared that the electrics in the room downstairs will be affected, and getting a long explanation from you of how it's a miracle that your son has even managed a shower so please don't go mentioning the puddles he left behind him.
You (and I include your DH in this) seem over-sensitive and inclined to over-thinking stuff. A multi-generational family holiday in a holiday cottage is bound to include moments when no one likes each others' behaviour and sometimes they say so. If you've really only once previously expressed a difference of opinion to your MIL then I regard that as an absolute miracle and possibly part of the problem.
As you've said, everyone has such high expectations of holidays this year — but holidays with teenagers are rarely easy. It was almost bound to end in tears.
The way we learn resilience is by watching our parents managing situations like this positively — and that means avoiding going into excuse mode or disappearing down the wormhole of blame and recrimination.
I have no idea what happened during your pitch and putt experience. What I can imagine is that your MIL, after 18 months spent mainly on her own, looked forward to an idyllic few days with her family and instead found herself living in a messy cottage with a grandchild who left water all over the bathroom floor and parents who seemed to regard this as something she should be celebrating rather than criticising. Later, after more tension and discomfort on a trip out, when she's in that 'I love you but I really don't like you at the moment' stage, you come up and tell her she needs to put her arm around her grandchild (or her son, who knows?) so that he feels loved. I can imagine feeling the need to get out of there too.
There's no right, no wrong, just a series of poorly-handled situations in which both sides might have done better. Move on how and concentrate on how to fix it.