QIf the enemy followers on the field are an unevolved Diamond Master and a different unevolved follower, can I select the latter when playing Rimewind?

ANo. You can select the unevolved Diamond Master with Rimewind, so you must select it.

QWhat does "Whenever an opponent is selecting cards for an ability, if they can select this card, they must select it" mean?

AIt means that when an opponent is playing an ability that requires them to select one or more cards, such as "Select an enemy follower on the field and deal it 2 damage" (found on Angelic Snipe, Bellringer Angel's evolved form, etc.), if they are able to select this follower, then they must select it.

Note that this ability is only valid when it's on the field. Also, when an opponent is attacking with a follower, they don't have to select this follower as the attack target.

QCan you explain "Whenever an opponent is selecting cards for an ability, if they can select this card, they must select it" in detail?

AAccording to the Comprehensive Rules, "if multiple effects that require a player to perform a certain action are produced simultaneously, and it is not possible to perform all of them, the player is required to select and perform as many of the actions as possible."

Let's look at some examples of this in action. In the following, X denotes a follower with "Whenever an opponent is selecting cards for an ability, if they can select this card, they must select it," and Y denotes a follower that doesn't have this ability.

Example 1: Your field has 1 X and 2 Y. An opponent is selecting "up to 1" follower for an ability. In this case, they must select the X.

Example 2: Your field has 2 X and 2 Y. An opponent is selecting "up to 1" follower for an ability. In this case, they must select one of the two Xs.

Example 3: Your field has 1 X and 2 Y. An opponent is selecting "up to 2" followers for an ability. In this case, they must select the X, but whether or not they select a Y is up to them.

Example 4: Your field has 2 X and 2 Y. An opponent is selecting "up to 2" followers for an ability. In this case, they must select both Xs.

Example 5: Your field has 3 X and 2 Y. An opponent is selecting "up to 2" followers for an ability. In this case, they must select two of the three Xs.

QDoes a follower with Assail have to attack this follower?

ANo, it doesn't.

QIf an opponent plays Crazed Executioner's On Evolve ability when this follower is in my hand, do they have to select this follower for me to discard?

ANo, they don't.

QIf an opponent plays Urd's On Evolve ability when this follower is in my EX area, do they have to select this follower and banish it?

ANo, they don't.

QIf an opponent plays Multipart Experiment when this follower is on my field, do they have to choose option 1 and select this follower?

ANo. Your opponent doesn't have to choose option 1, but if they do, they must select this follower.