"Conditional" is used to get your attention. That word should direct your attention to the notes on the plate which might specify different minimums based on the location the altimeter setting is from (some approaches authorize operations based on altimeter settings from a different airport with an increase in MDA/DA). Minimums may also be raised if certain navigation aids aren't available or operational.
It's just another aid to remind you to carefully examine the procedure because there's something special that allows you to descend to that altitude you need to be aware of.
The localizer back-course will be more sensitive because you're closer to the ILS antennae. On an ILS, the loc antennae are located at the departure end of the landing runway. On a Loc/BC the same antennae is used. Since you're flying towards the departure end of the front course runway (where the antennae are located) you're closer to the antennae throughout the approach.
On a long runway, it stands to reason that the BC would be much more sensitive than the front course.