@nwgal, if it makes you feel any better, every topic you touched on has been discussed already in this forum. The problems you mentioned are legion! Spelling and grammar are skills that improve with use. Keep reading this forum because successful shoppers often share their tips on report writing. There are also some current and former editors who post here. It's an easy way to learn.
Another way to improve one's writing is to do more reading. Some MSCs write up guidelines for shoppers. Google "mystery shop report writing" to get an idea of what is expected in comments and narrative. Keep in mind that every MSC's reporting style is different, but the spelling and grammar tips are universal.
As far as not being paid if the client doesn't accept your report, that's a sore spot for everyone. If we do the shop and submit the report to the MSC's satisfaction, then we should be paid because we fulfilled our obligation. However, if the client doesn't accept the report, then the MSC doesn't get paid, and therefore we don't get paid. Some MSCs will pay the shopper anyway, some won't. My point here is that you shouldn't take that comment personally.
Seven out of 10 isn't a terrible score, so don't beat yourself up about it. The editor who made a personal judgment must have had a bad day. That type of comment is unusual and uncalled for. You can play the little mind game I play. My first report for an MSC came back with the comment that I needed to their ESL class. I chose to believe the comment was meant for another shopper, and was attached to my report by mistake.
Last, the grammar police patrol the forum. Take their comments in the spirit of helpfulness in which it is offered.