proofreaders: they usually control grammar, but can also do other things
betareaders: when something is in “beta-testing” that means it’s about to be finished, it only needs to be checked and controlled by someone who is not the creator - so betareaders read a story after is finished, but before is published; they check everything usually
reviewers: usually after a story is published you can ask a reviewer to give “marks” to your story; the most used categories are choices, directing, grammar…
it should be! and if someone charges for it, don’t pay them! there are a lot of people and groups that will do it for free
both! groups mainly do it once you ask them, usually for only the first 3 chapters. but you can ask and search for a personal proofreader/betareader, it’s not uncommon
if you just want someone to proofread the story, you can send them the script (I do some proofreading sometimes, and personally I prefer if the writer sends me the script, since it’s quicker for me, but it depends). if you want a complete betareading (also directing and story development) you have to send them the link of your story, the one you find at the bottom of the writer’s portal
unfortunately after the story is published that link will get other people to the published episodes. I personally copy and paste the unpublished episodes in another story, and give my betareaders the link of that second story (unpublished), so that they can read it without any problem
and if you want to share the script, I suggest you use google docs, it’s easy to use and it does have different useful functions
I hope I answered everything, but if you have other question feel free to contact me!