since I have nothing better to do right now let me give you some facts:
1- what makes Halle a "B event"? they are both 250 series with the same prize money, players are free to choose whatever tournament they play between them, usually players prefer tournaments they can speak their language/close to their hometown and thats two reasons why Federer choose Germany.
2- Halle's surface (slow grass) is closer to Wimbledon than Queen's (fast grass), therefore the reasonable choice is to play Halle as a warm up for Wimbly.
3- Federer has been competing in Halle before anyone even heard of the name Nadal, you should question why Nadal choose to play at Queen's rather than come to the tournament Federer ALREADY plays in and wins every year.
hope this makes enough sense to you