The way I understand Delsin's power works is as follows: When he touches a conduit's hand he sees their memories around the time the manifest and learn to use their power. His body also mutates to be able to replicate the power they have. The way he uses it is a combination of his own style and the memories of the conduit it was taken from. For example, when Delsin gets video, he uses it differently from Eugene (firing rapid fire video clusters and not turning into a giant angel), however, when he creates holograms, they are angelic or demonic. These are probably the easiest for him to make because these are what he learned from seeing Eugene's memories. With time and practice it can be assumed that he will develop his own style of use for this as well. From what I can tell, Delsin needs to acquire a certain level of mastery over his new power before he can use previously absorbed ones again. This can be done naturally through practice and focus(like how he managed to use neon blasts) or artificially through absorption of ray field energy. As I understand it, the ray field energy allows him access to more skills already learned by the conduit he acquired it from (again this is why he uses angels and demons and not something more related to his personality). As for why later absorptions were weaker than earlier ones, I think this can just be explained as him absorbing too many powers in too short a time. His body probably just needed time to adjust.
Now that I've explained that, I would like to put forth a personal theory. I think that Delsin's natural ability will get stronger given time and practice as well as his artificially acquired ones. This is to say that eventually he may be able to use multiple powers at once and even switch between several without needing to absorb from a new power source each time. I say this because if this is true, then it is entirely possible that he can canonically be able to dual wield powers and switch at will. This wouldn't need to be retconning existing rules if they explain it as his power absorption ability getting stronger.
Thank you all for taking the time to read this huge post. I've spent a lot of time thinking about this and hope this can clear some things up or at least help you form your own opinions.