QIf I fail to summon the Steelclad Knight token with this follower's On Evolve ability because my field is full, do I still give each Steelclad Knight on my field +1/+1?

AYes, you do.

QI have a Steelclad Knight with Rush on the field due to this follower's passive ability. If I use the Steelclad Knight to attack an engaged enemy follower, and an opponent destroys this follower with a Quick ability, such as Execution, will my attack still go through?

AYes, it will.