Don't worry for the questions. This is a classic one, and it is my fault, It should be explained better.
For answering your request about 2.5 back compatibility, I am affraid that the B/C compatibility will not be handled forever, so the answer is no.
Cook will at the contrary follow the latest evolutions of Joomla and will always keep B/C compat minimum 1 year after the end of life of a version.
More than one year, it will be very difficult to maintain, and I do not process any tests anymore.
I know how difficult is to migrate a website, but if you don't migrate your Joomla, you are dead already.
Be carefull with that. Internet is a severe war, and your obselete Joomla is highly vulnerable, so don't do that.
Cook evolves slowly, so with some skills, you will often find a way to improve a component and copy your layouts and triads in an old component, but you take the more difficult way.
Migrate your websites. Once your server is compromised, it will not be only your Joomla to rebuild, but the whole server installation. Be aware of that.