Hi team,
I'm using MX linux 19.3, i downloaded MX with XFCE as default and then i downloaded KDE.
I'm very much impressed with KDE but i appreciate if i can know below things as well,
1. How do i make KDE as my default DE in MX linux
2. Shut down/ restart button are not showing in the log in scree (showing when i opt monochrome option, but it is grey color and cannot click/ or not working the icons)
3. How can i install KDE games/ applications in MX linux?

Please advise.

Did you try asking the MX Linux people? Since they provide the packages you use, this is pretty much in their hands.

Can I install kde apps like Krita, kpaint, games, etc using Mx package manager or synaptic package manager or muon package manager? Or discovery is really need to install kde apps?

Discover being the default KDE package manager it makes sense to use it, but of course you are free to use whatever you like.