Hello,
1) To install plugins go to Plugins > Install plugins and I don’t have any problems with that page.
2) Only XStore Core plugin supports the auto update from Dashboard > Updates. All the other plugins (bundled plugins https://help.market.envato.com/hc/en-us/articles/213762463-Bundled-Plugins ) you can either update via FTP or using file manager plugin. Also, you can disable+remove the plugin and install it again from Plugins > Install plugins and you will get the latest version of the plugin after that. To be able to use auto update option for the premium plugins you need to use the individual license, read the Envato post about bundled plugins.
Regards