though Electrum on mobile lacks some very good features like coin control, customizing fees, signing a message, etc.
You are wrong, I don't know where you get this statement but coin control and fee customization is available on Electrum for a very long time.
I'm not wrong, the android version of Electrum lacks a lot of features that is present in the desktop version, coin control and fee customization is available on the desktop version of Electrum, but not on mobile and you can confirm that yourself if you use Electrum on mobile.
No. If you read this link you posted above, you will see that the person was asking when coin control feature would be added to the android version of Electrum, because they enjoy using it in the desktop version.
Also, it seems that now sign-in message is also available now but I am not aware of it or can't find it in my updated android too so probably it's still not updated on Play Store but the sign-in feature on the electrum android has been implemented recently.
No, it has not be implemented, you cannot sign a message in the android version of Electrum, you can only do that on desktop.