For your first question, no you do not get to decide how many gems your choice costs. The number of gems is determined by the number of reads a story has. You can read this link for more information about adding gem choices.
For your second question, as a writer it is potentially good to add gems because you can potentially get into the payments program quicker by only needing 100k gem reads rather than 500k normal reads (in 60 days).
If you are going to use them, I would suggest using them every chapter since it only benefits you not the reader. What I mean by that is the benefit to you is getting into the payments program so you are going to want to include the gems in all chapters to keep getting gem reads. Having gems doesn’t really benefit the reader at all.
If you are going to use gems though, be careful how you use them. Make sure using the gem choice gives the reader a good experience (ie. don’t make them feel like they wasted gems). And don’t have the use of gems take away from the quality of the rest of your story.
I agree with @Purple_Ghost about needing them in each episode. At the end of the day, if you’re going to use them you may as well do it right so it helps you get in payment program.
I think certain stories have the kind of plot that suits gem choices (like mystery where you can pay for more knowledge or action where you can pay to bypass a certain challenge). If your story is just your average drama or romance, I’d recommend avoiding gem chapters as it’s kind of a turn off for readers. Not many people want to pay to kiss the love interest or to wear nice clothes when you can do that in another story for free.
It’s something you can add in. Just like you can make a gold choice box or a gem choice box. I see it in stories that use the point system. So they use it as a way to show you haven’t earned enough points to pick a particular choice
Dara has some great threads that explain it all…
This is a great one for the point system. I don’t think she’s made one on actually locking the choices, but I know if you search it, she’s answered it on other people’s threads.