I would say it was a german potato masher, troops were supposed to have operating knowledge of a number of enemy weapons for some obvious reasons..
Grenades were one of them, so it may not be unusual to see an allied soldier throwing one of these once an enemy trench has been captured... just not generally in the attack....
The bandoliers do look wrong though... the officer, at least that what he looks like to me seems to have a trench club....
I don't really want to debate yea or nay to greatcoats.
Did they wear them in April 1918? -yes.
Did they wear them charging across no mans land in 1918? - I don't know but unlikely.
They wore greatcoats but often soldiers preferred the leather jerkin and/or woollens* in the assault in rainy and/or cold weather. The greatcoat did trail in the mud and get weighty so the hem was taken up and the groundsheet was also used (like a cape) in wet weather but these would be for a more static role not for a sprint. The uniformity is not in keeping with the late war and I would have liked to have seen a mix of uniforms and poses. The bomber would have been better depicted with a Mills bomb as far more typical though an inert potato masher could be and was used as a cudgel. The cotton bandolier was plain though these look like cavalry/artillery types.
* 3 layers rule with the sleeveless jerkin as an addition