Sometimes that can be fun, but only if everyone at the table is onboard for a wild tangent. If the other players are bored as shit while the special snowflake starts a unicorn breeding operation, it’s time to use that No. And you, the DM, are included in that too; if your players want to drag you off to write every book in the library and that’s not fun for you, you have the right to say “hey maybe you should play the game I made for you instead.”

You decide that if an when the players make it a priority with their choices.

Addendum to the “Are you sure you want to do that” bullet: if a player ever does something that seems nonsensical to you, ask them what they expect to achieve by doing that. Understanding their motivation is often what resolves the miscommunication and/or allows you to steer them towards a better way to do what they’re trying to do.

Wow, just learned I’ve been missing the aoo rule for 20 years.

You could have the same spell OR the Counterspell spell. The benefit of taking Counterspell was that it could work against anything.

Spot on about the action economy observation though.