Hi!
I'm telling you Bissler you GOT to play Heresy. I think you'd like it.
I would think making an assault phase that goes last, thus no major rule changes, is the way to go. Resolving it immediately introduces a host of problems and unforeseen consequences.
Regarding the other problem, it is very "game board" dependent. For example my board is FIVE feet wide (over 150 cms), so this is never an issue because the separation is over a 100cm. On a smaller width game board it is most definitely a problem.
One idea I had for a long time for net epic is "no deployment". In other words nothing starts on the table and EVERYTHING needs to move onto the board on the first turn through designated deployment zones. Makes for an interesting game, because there is no "sit and shoot" on that first turn (no first fire).
You are indeed correct to state that is why they made ranges shorter in later epic rules. It forces you to move to get within firing range. I never liked that solution much, it strains my suspension of disbelief that a defense laser can not fire across the battlefield, it supposed to shoot spacecraft in orbit!
There is only so much you can do with the old second edition chassis. Certain things (suppression, one activation resolutions, etc), that either don't fit well or are difficult to balance. This is why I don't try to make it into what it is not. It is what it is, love it or hate it.
That's why I made heresy.
Primarch