@Omni5876 Cole is under the impression that if he doesn't get a decent following soon people will start thinking he's weaker than the spear, and bow. That is a killer to his confidence level. He doesn't not believer Fer so much as while he's leading Fer still is older, and seems to have carried favor with both of the individuals who seemingly will be long time allies. Its the concept if Fer leaves he'll basically be on his own. So while they're following Cole they're still technically their own party, if Rivia officially joins Fer, while Cole until Edusa joined him would've been travelling off to god knows where by himself if he couldn't get Marcino to help who also is giving info to the spear, and bow.
As for absorbing magic I don't think so until Fer kills something that can and puts the components into his shield like Naofumi does. If he wants to learn magic he can do so quickly after the brief 5 day timeskip i'll be doing soon, assuming no one wants to dog the king, rin, cole, the nobles, or edusa/shay. The shield hero is made directly to debuff enemies, or shield allies. He can heal, chain up enemies, and otherwise protect/hinder individuals anything Naofumi does and you can provide solid evidence to that Fer can learn to do in the given time Fer can do as well.
Edit: Also i'm not sure if you remember but Auriel officially swore herself to the service of the shield at the start.