If you edit file admincp/rssposter.php, around line 363 is the end of the list of choices:
Code:
36000 => construct_phrase($vbphrase['x_hours'], 10),
43200 => construct_phrase($vbphrase['x_hours'], 12),
), $feed['ttl']);
If you want, you can add some lines to that, like:
Code:
36000 => construct_phrase($vbphrase['x_hours'], 10),
43200 => construct_phrase($vbphrase['x_hours'], 12),
86400 => construct_phrase($vbphrase['x_hours'], 24),
172800 => construct_phrase($vbphrase['x_hours'], 48),
), $feed['ttl']);
(The first number is the number of seconds in that many hours). I looked at the rss poster code and it looks like that's the only change you should have to make, but I haven't actually tried it.
As for your second issue, I think some feeds contain something that causes an error in the rss poster code and it crashes, so the other feeds don't get processed. I helped someone a few months ago with a similar problem, but I never heard back from him to know if it worked. Maybe later I'll have a chance to dig it up and see what the fix was.