I'm very confused (nothing new for me) and probably won't really understand things until we get to try it, but what would be the actual mechanics of attacking and defending? So lets say Tritan wants to attack a district owned by STB, STB gets a message they are being attacked and they get to set a time for the battle? Do they have a fixed time window?

The way this is sounding is, all you need is 16 awesome players and you're golden, that would be a real shame.

IMO the attack/defend system should work like this. Corp A attacks district owned by Corp B. Corp B now has 1 hour to muster its best defense. Both corps can pull in anyone willing to fight for them through squad mechanic. Any members of corp and the corp's alliance can join the battle individually. This gives corporations incentives to make allies and have their back watched at all times and to recruit from all parts of the globe.

Defenders have advantage of knowing terrain and having preset installations.

Attackers should have the advantage of timing the attack.

I should clarify 3. No you cannot cancel the attack. Think carefully about how many clones you want to attack with.

You can however move clones from district A (which you own) to district X (which someone else owns) and then move clones from district B (which you also own) to district A. Once another district attacks district A though you cannot move more clones to it.

The Elephant in the room map question:-

Will this be released with a fully implimented - terrain unique to each planet - dynamic/procedurally generated maps

Or will it be ashland, line harvest, manus peak, skim junction, and the wards with the same old fishbowl terrain and randomly generated structues (what we sort of have now) with the SI as a central feature.

I think owning all, or most of the districts, should allow you to name the planet. Makes it a bit more of an achievement. And no one can re-name it until they take the majority of districts. Would be great fun.

The Chest beating will commence

There is possible gameplay problem with this system. It prioritizes clone count over depletion over MCC destruction since MCC destruction only does 100 clone loss, while a clone count depletion has the potential to completely win a district, or to least make the enemy take a bigger clone loss. I fear Skirmish will become a glorified Ambush battle.

There needs to be more game modes or objectives associated with planetary conquest or else it will only be a glorified team-deathmatch. Here are possible ways to add complexity and variety to the actual battles:
Attacking/defending clone storage to destroy clone supply.
Attacking/defending to steal clones by the hundreds.

The defenders get access to all of the clones on their district. Whether they choose to use them all up is up to them

If a defender chooses to hold some clones back and loses then what happens? The attacker wins the district but what happens to the reserved clones?