This is long but I'll do my best to answer everything. Also, I'm a bit drunk, so bear with me.
First of all, your brother is a good luck charm. If your two player matches are fine, and your single player matches aren't, then it's just luck. You get connected to people with poor connections. All luck from what it sounds like.
The goalie isn't 100% all the time, and he is sort of a retard. If he's diving out of the way of goals and they're still going in, it's just luck (or wind if you're on Thunder Island.) Now, if the goalie is swatting at the ball, or doesn't even make an attempt at the ball, that's usually a trick shot.
Thirdly, with two toads and a DK just flipping over you, the best thing to do is wail on the toads, make him prove to you he can score some other way. Knock them into the fence every chance you get, don't worry about his items. Normally, when you take away a dekeing teams strategy, they panic and then you can play your game.
Lastly, Daisy is way more popular than Bowser Jr. And that's because they're 9/10 the best captains. Daisy's special is practically a free goal, Diddy's Red Card is cheating, and DK is the best out of the power shooters because of his special but, more importantly his deke (it's a bit shorter recovery wise than Bowser's and Petey's.)