GENERALS

Can a portuguese general be attached to a british unit, and vice versa?

All leaders and units for the nations you command are considered friendly. Unless the Scenario Special Rules prohibit it, one nation's leader may attach to any other national units that you command.
And all units on the same side as such a friendly Leader, benefit all bonus of this Leader.

Can a Leader fullfill any objective hex victory condition?

All kinds of "Objective Victory Banners" can only be fullfilled by units and not by Leaders (remember rulebook page 7, “A Leader is not a unit, and is represented by a single block." except, the scenario notes described it explicit otherwise.

Does a friendly leader in one of the two hexes that cavalry need retire into block the action? For instance, in the example on page 16 of the rules, if there was a friendly leader on either of the two wood hexes, can the cavalry retire and reform?

A cavalry without an attached leader may Retire and Reform one hex onto a hex that contains an unattached friendly leader (i.e. a leader alone in a hex). The leader is immediately attached to the unit and the cavalry unit does not have to retire and reform its second hex (it must stop then on Leaders hex).
If a Cavalry retire then
1. Cavalry without attached Leader stops onto a hex with a lone friendly LDR
2. Cavalry without attached Leader may not move through a hex with an enemy LDR/unit or friendly unit (basic rules page 7)
3. Cavalry with attached Leader may not moves through a hex with an enemy LDR/unit or friendly unit (basic rules page 7)
4. Cavalry with attached Leader may move through a hex with a lone friendly LDR (basic rules page 7)

Can a retreating Leader escape through a hex with a lone enemy Leader?

Yes

If a leader can escape through a hex with a lone enemy Leader, how many dice would the lone enemy leader roll?

The leader when alone does not battle, so no dice are rolled.

LEADER ESCAPE

Q: How do i determine the number of dice during a Leader Escape roll.

A: By a leader escape roll, no Terrain- and Command Cards rules apply.
Only own skills of an unit or special formation limits (Square for example) can increase or decrease the number of battledice.

Example 1:
Q. A Leader escapes through an enemy (3Block) Heavy Cavalry, which is in a town hex. How many dice the HC rolls to hit the leader?
A. 4 dice

Per the Leader Escape Procedure Rules: Move the leader onto the enemy hex and allow the enemy unit in the hex to battle the leader. The attacking unit uses its normal number of melee combat dice. In this case Heavy Cavalry will roll 4 dice. The leader does not benefit from terrain in the hex.

Example 2:
Q. If a leader escapes through an enemy Infantry unit in square formation, how many dice rolls this infantry unit roll to hit the leader?
A. 1 die

Per the Leader Escape Procedure Rules: Move the leader onto the enemy hex and allow the enemy unit in the hex to battle the leader. The attacking unit uses its normal number of melee combat dice. In this case 1 die, because one die is the number of dice an infantry unit in square rolls in melee.

LEADER CASUALTY CHECK

The Expansion 5 (Marshals&Tactician) rules states:
With the Generals, Marshals &Tactician rules, a Leader alone in a hex, may now be targeted in ranged combat (targeting in melee combat has always been allowed). When a Leader is alone in a hex, the attacking enemy unit rolls its normal ranged battle dice. Two saber symbols rolled will score a hit and eliminate the Leader. If the Leader is not hit, the Leader must retreat 1, 2, or 3 hexes. Flags rolled against the Leader have no effect on the unattached Leader. So a lone leader can be attacked in range attack and 2 cross-sabers are needed to kill him, which requires an attack with at least 2 dice!
Q: Is it possible nevertheless, to attack the lone leader in range combat only with a single dice attack, which can´t kill him (because 2 XSW are needed), but to force to retreat him after the attack?

A: A single die range attack on a lone leader is not possible.
The following note will be added to the updated rules on page 6, under Leader Casualty Check. Note - A lone leader may not be targeted when rolling less then two dice. Also note - sabers normally will not score a hit in range combat, however, firing at a leader alone in a hex is an exception to this rule.
Richard Borg: 2017-12-05