Firstly it can be used in gvg, championship and tournament matches. If you have example a 10 star average team using a 50% match bonus would make your team 50% stronger in all fields therefore your team will be 10* + (50%) 5* = 15 star strength. Using 30% match bonus will be 30% of 10 star team therefore your team strength will be 13%. Hope this explains it and yes mb is the devil in disguise
In guild challenges only the challenging team can use match bonus, it does not apply to the defensive team. When applying match bonus you are paying to make your team stronger and the money will be deducted from your balance at the start of the games.
I'd disagree. Simulates high-energy games from your players, like they're motivated to win (think division rivalries), and it's an extra way to spend cash when you have too much.