Not sure for certain, but i think that the GW products are more based towards the films than the books, for example Gamling the old in the books is with the men of Helm's Deep, while in the films hes with Theoden, so GW made him the banner bearer, in the books his banner bearer is Guthlaf.
While Eomer is third marshal he commands the East-mark not the West-fold. I think Erkenbrand is the lord of the West-fold
Info about Grimbold, spoilers.
Grimbold was serving under Theodred until the first battle of the Fords of Isen when Thoedred was killed. Not sure who he served under after but i'd assume he under Erkenbrand or Elfhelm. After the second battle of the Fords of Isen he went with Erkenbrand to break the siege of Hornburg/Helm's Deep. He was killed in the battle of Pelennor fields.
Yeah, the Helmingas should've gone to Erkenbrand but Erkenbrand already has West-fold red shields so they for whatever reason decided to give it so Grimbold, probably because they used Gamling as banner bearer of Theoden with the Banner of Rohan so there was noone left to take it or something.