They get around this by using a version of Lenin's definition of imperialism. Lenin characterised imperialism as, in very simple terms, the way that powerful capitalist countries exploit poorer countries (or straight up colonies, especially considering the time in which Lenin was writing). Of course they use a version of this that specifically defines it as just literally anything the countries they don't like do, leading to ProleWiki insisting that Russia is not imperialist but Liechtenstein is
Similarly to how 'reactionary' just means 'bad' to these people. For the record a 'reactionary' is somebody who wants to restore a previous status quo. It's not inherently good or bad.
Reactionary, in political science terms, is the opposite of radicals. Radicals are to the left and reactionaries are to the right. In practice, people on the internet tend to use radical for both, but I wish the distinction was made more clear.
You are correct. Radicals want things to change in an extreme way, and reactionaries are just that, reactionary to change. Not sure why you got downvoted for knowing what you are talking about.
Language does change, though, and often laymen use words differently than subject matter experts.
No. Radical is simply somebody who wants to fundamentally change society. It's not the opposite of a reactionary, in fact, you can be radical and reactionary at the same time.