Shelter Island Heights
1) Can the Mud card be played to prevent withdrawing or intercepting?"
The Mud card only affects movement allowance, and therefore only "true" movement, not interceptions or withdrawals (for which the concept of movement allowance is irrelevant)
2) Can a player play two Forced March cards to increase the movement ratio of his intercepting or withdrawing army by 2?
3) Can a player play two Forced March cards to increase the movement allowance of his army by 2 (before his move)?
If yes, does this add extra march attrition for the second Forced March card?
4) 12.5.7 and 12.6.8 don't demand to check for march attrition during winter when withdrawing or intercepting. But 188.8.131.52 demands this for retreats. Is that correct?
The first two should have mentioned winter too, as the general rule (12.3.1) makes it clear that an army entering any area in winter (whether through movement, interception, withdrawal or retreat) is subject to march attrition. I wish you had been a proof-reader!
5) If a player withdraws or intercepts with the help of a Forced March card, can the other player himself play a Forced March card (after beginning of the movement of his army) to prevent the first player from withdrawing or intercepting?
No, because escaping interception is not listed as a use for the FM card, onlu movement, withdrawal and interception.