Information about colored folders support is here: How to color email folders - #4 by cyberzork
You can open multiple instances of eM Client starting from version 8:
https://support.emclient.com/index.php?/Knowledgebase/Article/View/183/0/can-i-run-multiple-instances-of-em-client-can-i-have-mail-and-calendar-in-separate-windows
Unread and total summaries are shown at the top of the client once you select the folder, but not in the folder list. I feel like if it had to get counts of every folder consistently it might make client operations slower, such as the same if you had a file explorer calculating all the sizes of folders in the folder listing. So calculating on click of the folder might be a more efficient way and why they selected that method.
Myself, I would rather the important features be integrated directly rather then through plugins. I have always felt plugins can effect the security and reliability of the client in question negativity, which is why I usually use a web browser with the ad blocker built in and minimize the use of plugins on clients that are used heavily day to day. A big problem I have with plugins is they are not always done well by 3rd parties and once they loose support become security risks and break with client upgrades. That is not an issue if those features get integrated directly, and I think eM Client tries to implement as many features possible balanced with reliability fixes through the ongoing client upgrades.
Lastly, as a prior poster mentioned, the Android and iOS apps are coming soon. I think they want to make sure they get the desktop client in a good place before throwing the mobile clients into the mix. Right now I am sure their focus is getting the v9.2 released as soon as possible.