You can trade 3 first rounders. Not sure you'll get off that easy in the unlikely event.if essendon didnt match for daniher wce wouldnt match for allen.
matching and forcing a player to stay is unprecedented. matching because you can get a better deal out of the club acquiring the free agent than the compensation formula would otherwise provide is why a club would match.
wce would likely get pick 2-4 for allen which is way more than what brisbane could trade if matched.