A: Great questions!

1. You can add 1 channel in tier 2. But this is only needed if you plan to write scripts with Subscribr. In this case, you can delete a channel and then add a different one, but doing so will delete any scripts you've created.
2-4. For video analysis in Ideation Chat, there is no limit to the number of channels you can analyze. Just mention @ChannelName in the chat and it will retrieve it without you needing to do anything differently. No credits are required for the analysis features.

By the way, since you are interested primarily in the analysis features, I would encourage you to review the new Intel features that were just launched, since these features are all about analyzing channels and videos. Details here: https://subscribr.canny.io/changelog/subscribr-intel-is-here. Note that Intel requires Tier 3 or above.