Had trouble firing the event for destroying 6 Allied ground targets in the mission. The game would only count the anti-tank gun as destroyed and not the scores of British troops I mowed down.

This is a known issue, that surfaced in the last patch. For the moment it seems, that you can´t progress any further without modding or using cheat codes. It should be fixed in the next patch, see here:

http://www.slitherine.com/forum/viewtopic.php?f=372&t=85476&start=20

There is, however no info, when the new patch will be available, and what new bugs you may encounter.
Order of Battle is a great game - but the way they handle bugs, patches and especially how they communicate about it - it is frankly frustrating - to put it very mildly.

I agree with what your saying, the games great but there's plenty of 'old style' thinking and sluggishness in general Slitherine games.

Just a (late) update, this is no longer a problem and has been fixed.