My yard is xeriscaped with river rock. Bark mulch keeps blowing onto it. Would this broom sweep up the mulch while leaving the rock undisturbed?

I use mine to brush leaves out of a pebbles walkway and it does a good job. I would expect it to perform with bark mulch in river rock, but the bark would probably need to be dry and light weight.

Will this scratch Trex decking?

I have Trex decking. I have used this broom on it and NO it does not scratch it... You will LOVE this broom! I have given friends this broom & they sing its praises too! IT makes clean ups SO much EASIER! You will not regret this purchase!

Part of the wooden cap holding the bristles was missing when I received the broom. Will that affect its performance/durability?

My latest broom is now over Maybe 6 years old ...still looks & works Like new. The cap should be circular and cover all the twisted bristle stems that make up the broom....I would return it ...because That cap keeps the broom intact . I have given many of these broom to friends who garden.... That cap you are talking about has always been intack.... Just the best garden/out side broom ever!