I can see both sides. I think you are all being unreasonable and that a lack of communication and organisation from BOTH parties have led to this.
I can understand you wanting to go round, collect momento's and reminisce before the house was sorted out ready for sale. It's not something everyone wants to do, but as you had asked they should have been more considerate. I can understand you being upset at that and it's not unreasonable to be so.
However, you did not say when you intended to go round to your DGM- you assumed it would take longer for them to get round to it. There may be any number of reasons why it was done quickly- perhaps your DM wanted to get it over and done with, rather than having it hanging over her? Maybe this was the most convenient time? It's not an unreasonable timeframe in my view.
Did you offer to help or did you expect your parents to hold off doing the work of clearing until it suited you? If you aren't going to help with clearing, then I think you have to accept the timetable of the ones who are doing the work TBH. I think you should have been clearer about when you were planning to come.
OTOH, they could have let you know they were intending to clear house on x date, so if you wanted to come you would need to do it by then. They WBU not to do so as they knew what your intentions were, it was thoughtless.
I think you were unreasonable to have a blazing row over it- you are obviously (and understandably) upset that your gran has had to go into a home but has it occurred to you that your mum may be equally upset that this has happened to her mother? You talk about your dad not listening/taking into account your feelings but don't mention your mum or what her thoughts/wishes/feeling might be! Can't you see the irony in that?
I know when you are angry it can be hard and I am genuinely not trying to be unkind but it's not all about what you want/what you need, in the same way as your parents should have been more considerate of your wishes. I think a bit more empathy and compassion on both sides is needed.
If you parents are generally good people then I think it's not worth the fall out. IME these kinds of falling outs have a habit of sticking and things are said that aren't meant but can't be taken back. I doubt it's what your DGM would have wanted- her daughter and grand-daughter fighting. And poor relations between POA's seldom leads to an easy life for anybody.