Followers could already use emotes with Channel Points
Followers already have an ability to use emotes by using channel points in channels they actively engage in. This feature I feel would clutter an already hard to navigate emote viewer. It also makes it so people would just follow for an emote and not engage with the streamer or their content. I feel the channel point redemption for using emotes is good enough for this as it encourages viewer engagement with the channel in order to use them.
On the second point, "follower Emotes can only be used in the channel in which they are unlocked" according to https://help.twitch.tv/s/article/follower-emotes