Leeds wrote:1. How do I select a Main Army in the WEGO method?
I don't understand the question. Main armies can be changed (button).
Leeds wrote:2. What is the difference between an HQ's Command Radius and Intervention Radius?
I understand that Intervention Radius is the range from a combat hex that the HQ must be to commit reserves for a breakthrough AND also the range a detachment can be from an HQ unit?
Yes, intervention radius is used for reserves commitment. Command range is used for stacks commands. Stacks out of command move slower or even do not move (commaind check failure).
Leeds wrote:Can a detachment be 5 hexes away, thus in Command radius?
It depends on the HQ command range.
Leeds wrote:3. If I am using the AI Battle advisor will it automatically begin a siege?
Sieges start anyway when conditions are respected. AI Siege advisor simply takes decision on behalf of you, when you are in the Siege window.
Leeds wrote:4. How do I build a fort or trench manually?
Fort are not built manually. They are given by scenario setup. No way to build fortification in this game.
Trench level is increased by the adoption some key technological advancements (flamethrowers, land mines, land mortars,...) when you enter the Firepower doctrine.