Etiquette for optional rules vary from group to group, but the rulebook says that they may be used “if all players agree”.
Just to be clear, this is only an optional rule in Classic. In Revised, it’s a standard rule.
#1 Using LHTR rules if there is a transport that started the turn with 2 inf aboard, Can you unload 1 inf in combat and the other in noncombat (same territory).?
#2 If a transport was forced to retreat from an amphibious assault, can it unload in noncombat move (units were loaded in combat move)? - LHTR rules as well.
The answer to both questions is “no”.