one: Not sure really, sometimes it gets on my nerves because it sometimes can be confusing or lowers credability if it's done in fanfiction. Probably for some people it's easier to refer to Jonathan as Sebastian because they share the same name and in conversation mode it's easier for other people to know which one you're talking about (though cos Jace is called Jace it's not really a good excuse?)

two: not sure really, dont tend to think of them beforehand, because im always disapointed by the actors because they never do it completely like i imagined the characters to be

It gets on my nerves. I feel like the name represents the person and with the name comes all the horrible things Valentine wanted him to do. By still calling Jonathan Sebastian, it makes Jace Jonathan still. Which still makes him like the demon baby. Sorry, I'm not sure if that makes sense.

it does haha and yeah wild-bby i feel like that too about the actors i mean so i try hard not to get my hopes up on it