It helps people writing new lists but at a cost to old lists, ie Orks who I think are the only existing army who take advantage of this infrequently used rule.

Should we be making Forts any worse than they are and the Ork list less quirky and flexible?

No lists are broken by this, if it ain't broke......

The only exceptions are new lists, The Ork Horde list was one of the original 3!

Should War Engines count as a single Unit for garrison purposes (meaning you could Garrison an 8DC Titan by adding a single scout unit to the formation), or should it count as its number of DC for Garrison purposes (meaning you could Garrison an 8DC Titan by adding 8 scout units to the formation)?

RAW implies the former, but RAW also says that War Engines should only roll 1 dice when Teleporting, and that got FAQ'd to say they roll 1 dice per point of DC.

For most rules purposes, a war engine counts as being ‘worth’ a number of units equal to its starting damage capacity, so a war engine’s starting damage capacity is also used to work out how many dice it rolls in an assault, how easy it is to pin and so on.