For (1), see T6200. This will probably be fixed soon-ish.

For (2), see T5029. We have no plans to pursue this in the forseeable future.

@epriestley, for (2):

• interesting, why this field is made editable if changing it have no effect on revision it self
• can you please point in place in the code, that is responsible for context setting from arc diff command?

• It affects the revision's visibility, Herald rules, etc.
• We don't fill it in, we just generate a diff with -U9999999 or similar. Here's the command in SVN: https://secure.phabricator.com/diffusion/ARC/browse/master/src/repository/api/ArcanistSubversionAPI.php;52947cfd92b331916adffa1ff3233546446c881b$307

So you pre-generate context by arcanist based on code of the working copy. And Show 10 more lines doesn't really point back to changed file it's just text that comes in with a patch.

I must say I'm impressed. Never would have thought that it works that way.

Yeah, we're basically cheating. It's way easier to implement that way than trying to look things up in the repository, and it works if the repository isn't tracked or all the context is unpublished.