Try SD Client free for 14 days. If it has put your mail in order, stay for less than a coffee a month.
After 14 days you choose monthly or yearly. If you don't, you simply lose access to SD Client.
No. Your mail always stays in Gmail. SD Client only shows and sorts it, so your Gmail is exactly as it was.
All of them. The price is per person, not per mailbox.
Yes, any time. The monthly plan stops at the end of the month, the yearly one at the end of the year.