I attended one of those talks at Google on Youtube wherein they call different channel owners (creators) and help them build their channels. Some of the questions you are asking were covered in those talks. In case if you have your own channel and have uploaded 100+ videos, someone from the Youtube team would have contacted you regarding the same. Having said that, here are my thoughts -
The tags in Youtube are used to relate videos and help your audience find related videos when viewing one of your videos. What is used by searching engines on Youtube videos is titles and descriptions.
Why you should use regular titles is to indicate the presence of a series. Say for example I am searching for a recipe on how to create orange squash, and I find this awesome video having a title Summer Coolers - Orange Squash (this is just an example), then I would realize that the entire series would be on different cooler recipes. I would be more inclined to follow the channel ... the idea is to get the user to follow the channel.
The analogy that most creators draw here is that of a TV channel. If you have to run a schedule of your TV channel then would'nt you want that the different series running on your channel have a brand recall of their own (sometimes so strong that)? Hence the series names. Would advise you to take a look at Creators Academy.