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Personally, I have no problem with fire from a BUA being at full range. The protection afforded to the defenders and the chance to rest weapons may well cancel out the fact that volley fire was problematic - and of course firing by volley doesn't really increase the range of a musket.
If you feel crossing fire is too effective, I suggest applying the -1 modifier for a difficult target at all times when using crossing fire. In practice, I often find the fleeting targets of crossing fire end up deserving this modifier anyway.
As for Lobositz, consider the following quote on the initial Prussian cavalry charge given in Duffy's By Force Of Arms:
"The cavalry was the cream of the Prussian mounted arm, and the Austrians let it advance without opposition, 'but after they had lured the enemy close enough, not only did they open up a heavy fire against the flank from the village of Sullowitz, but all the Hungarian infantry (probably Croats), up to now hidden in the sunken roads and ditches in the plain, came to life all of a sudden..." (p.25).
The flanking fire from Sullowitz is mentioned in most accounts of the battle, muskets as well as artillery (see the Wikipedia article). Don't forget the full 30cm range includes battalion guns. If you prefer to limit the fire to 20cm, you are of course welcome to do so. In my opinion, the rules as written represent what occurred on the day of battle reasonably well.
No, it's a bit easier than that. When the national table says deduct half a move to move backwards, it means you can just move the unit back half a move, keeping it facing forward. See also the bottom of p.19.
My guess is that your questions relate to the Marlburian adaption of HoW by Roland Nickson, as I don't recognise the rules you mention. I would suggest you post on the Marlburian section of the forum and with luck Roland will set you on the right course. I'm afraid the Marlburian rules are entirely down to Roland and I have never used them.