Well lag will always be an issue. In such a mission you'd really need to manage lag via scripting. Remove dead units, create the straggled German troops later on in the mission - you get the gist. With the Higgins, don't send 15 in at once, I know that would be more historically correct, but, for reason of game limitations send in 5, create and send in another 5, and so on. I mean even in MOH:AA there are only 5 Higgins but it really isn't noticeable and doesn't seem to bother anyone. There are alot of ways to work around lag, they are just a few. And make sure you don't unnecessarily take up PC power. If the player is in a Higgins farthest right, and you intend on blowing up a Higgins farthest left, don't fill that one with men, because it's just a waste as the player will not see it. The explosion and debris they see later is more than enough.
As for MG fire, well, best bet is to reduce their skill level and maybe set up a script which gives them definite targets. For example, give them the command to shoot at object A, B, C, B, D, Player, A with a few seconds in between and then loop it. That way, only once in the cycle of about 30 seconds to a minute does the player get shot at, and if that doesn't kill them a straggler will or they may walk straight into the line of fire.
Anyways. Alot of ways to work through any problems. So if you happen to encounter any while you make this make sure you ask around because it's about time we saw a half-decent beach landing mission. ;D