how can i reopen the same file by clicking on the icon on the taskbar without using the back up option …

As Notepad++ (NPP) uses the same configuration settings each time it loads (by default) and the backup settings are stored in these configuration files you cannot achieve what you want (directly).

However there is a command line argument (see ?, then Command Line Arguments…), look for -settingsDir=. I’ve never used this and have no idea on how well it might work for your need. However if it does it would mean you could specify an alternate set of configuration files for running NPP which have all those backup options turned off (at your risk). It would also mean you would have to create a second icon on the taskbar which has the command line parameters to suit this alternate option.

Also read the announcement for v7.9.2 here which was when this option was implemented.
The corresponding github page here has more background information.

As for your second question, can NPP cause high disk usage? Possibly, if you have a lot of large text files loaded and the backup settings set to a low value (backup every xx seconds).

I should at this point reference the online manual and the information about backup here
Also you should read one of the FAQ posts called Periodic Backup vs AutoSave Plugin here.

There has been much debate over which settings are best, and in the end it really depends on what you prefer and what you can afford to lose if the application (or PC) crashes.

Terry