Namor’s ability centers around the Inactive player choosing to declare blockers. A force-to-block ability does not allow an opponent to choose a Character die to block or not. The affected Character die must block, unless there is an ability specifically stating that Character die cannot block (e.g. Doomsday: Villainous Monster, Nightwing: Protector of Bludhaven), which is covered by this rule:
3.1.14 If two abilities directly conflict, the ability stating you “can’t” supersedes the one that states you can or must. This is usually noted in abilities that state “if able” and is intended to let players know that an ability can't force something to happen that is disallowed.
There is no conflict between a force-to-block and
Namor’s abilities. If a Character die is forced to block, the cost required by
Namor’s ability is not paid, per Rule 3.1.21 (cited above).
Namor’s ability does not infer that none of the Inactive player’s Character dice cannot block, nor does it infer that the Declare Blockers sub-step cannot occur. Therefore, Rule 3.1.14 is not applicable.
Namor’s ability means that if the Inactive player
chooses to declare blockers (i.e. choosing to proceed with the Declare Blockers sub-step), they must pay a cost.
We can see the confusion with the Inactive player choosing a blocker through a force-to-block ability (a counter to
Namor’s ability, as you pointed out and an unorthodox, but legal, way of making a Character die a blocker) versus that player directly choosing to declare blockers. The Inactive player is choosing to pay a cost for the force-to-block ability, which would cause the Declare Blockers sub-step to occur, vice specifically choosing to proceed with the Declare Blockers sub-step - a fine, but relevant, distinction.
{NOTE: this paragraph has been superseded – see below.} Again, as was stated in answer to the first question, if the Inactive player decides to declare additional blockers with the Character die they forced to block, they must then pay the cost imposed by
Namor’s ability, regardless that the Declare Blockers sub-step will already occur because of the force-to-block ability.