2) How can I refresh the errors and discards columns?
I scanned a Cisco switch and it showed errors on several interfaces, then I cleared the counters of the interfaces in the switch and rescanned with switchport mapper. It still shows the same number of errors.
The MAC address database is kept in %APPDATA%\SoftPerfect\Switch Port Mapper. You can simply delete the .DB file from that folder: this will prompt the creation of a a new database.
Regarding the error counters, it looks like the switch might have cached them somehow or hasn't updated the data that is reported via SNMP. I recommend checking the current contents of columns ifInErrors and ifOutErrors in ifTable with any other management tool, or rebooting the switch if possible.