How I can show private chat history on the Web Admin or/and
export history by specific date/time into the file? (I use Mysql for archive messages)
ejabberd doesn't provide any specific method. I guess you can use SQL queries for that.
How I can configure MUC archive messages into mysql DB?
If you mean the archiving feature implemented in mod_mam, you can enable archiving in a room configuration (using a Jabber client, or enabling that option by default in ejabberd.yml)
How I can export users passwords from encrypted mysql DB?
If the passwords are encrypted (using SCRAM by ejabberd), then they can't be decrypted.
Why I can`t configure MUC rooms and users nicknames on the Web Admin?
Because the MUC pages in WebAdmin are very basic, only to view list of rooms and a few details, as you can see.
If you install mod_shcommands from ejabberd-contrib, that module provides a new page in WebAdmin to run ejabberd commands, so you can run commands like change_room_option and muc_register_nick