I don't think he was being naughty at all - Natasha failed to grasp the pricinpal of the task which was to reinvest your money back into products that were selling and continue to sell them.
Had she done that on day two - her sales would have been higher than what they were.
Jim & Susan won the task for her.
Lord Sugar's fine was a warning to Natasha because he probably knew she wasn't coming into the boardroom.

Highly likely.
Unless he wanted them to lose (and the other team to have the treat) and got his maths wrong......

Either Melody or Helen could have gone, to be honest, based on this task, but it was probably fair to give Helen another chance based on previous performance.

Hmmm, I'd give her more credit than that. The door-to-door selling was always going to be a disaster, but as far as I could tell, she took a flyer herself on choosing & buying those bracelets, sold most/all of them on her own at good margin (£5 sale on £1.50 cost) and wanted to get more, which was exactly the "brave" attitude LS was after. The impression I had was that Natasha thought Susan would fail horribly, giving her a specific opportunity for blame in the boardroom, but it didn't work out that way...

I thought it was unfair of Lord Sugar to deny all the winning team a treat when it was due to Natasha as team leader that they had failed to reinvest.

I wonder if there was some organisational problem regarding the trip to Goodwood he described and he was just looking for an excuse to cancel.

Seems suspicious that the one week they actually had a treat worth bothering with, he cancels it. Bet he was just making up what it was, and it was actually just supposed to be two free burgers each in McDonalds, which is about as good as most weeks!