Old paint questions

dunroving

[Just to be clear, topic is questions about old paint, not old questions about paint!]

As part of my clear-out and spruce-up of my house for sale, I am dealing with various left-over paint, some of which is full/unopened.

Used/old/useless indoor latex paint disposal - some previously-used paint I have been able to leave open so it solidifies, and have instructions from local council as to what to do with it. No problem there.

Other used/old/useless indoor latex paint, the cans are almost full and after being open for a week, still show no signs of solidifying. I had read online a suggestion to put dirt into it, in order to solidify it. Can anyone advise on this? It's an "irreversible act", so don't want to do it unless it works! I also saw a suggestion to mix 2 parts cat litter to 1 part paint, but dirt is cheaper!

I the opened an unopened can of Cuprinol One Coat Timbercare shed/fence paint, which is maybe 3-4 years old, with the intention of painting the shed for the new owners, so they don't need to worry about it for a couple of years. It had separated somewhat so I used a paint mixer to mix it. However, the result is still very watery, and definitely not "one coat". Does anyone know what the shelf life typically is for that sort of exterior shed paint? It seems to me if it hasn't been opened, it should be useable, but it's not very satisfactory. I figure I will just keep using it until it runs out - no harm done (hopefully!)

Lastly, I have 20 litres (4 x 5 litre cans) of Dulux matte latex wall paint, also purchased about 4 years ago. I haven't opened it yet as I plan to use it at my new house - should that be OK? Is it best to take it to a paint shop and ask hem to mix it on their shake-a-bake machine? Again, online it says it should last up to 10 years but wondered if anyone has experience of this.