That's not a schematic. It's also not similar to the above URL's. There are numerous different suppliers of the DI-G650, 650+, etc. Which picture resembles your card?
Yes. The odds are better that one or the other antenna will NOT be sitting in a null (where the incident and reflected waves cancel).
It's where the radio section switches between the two antennas and trys to determine which antenna is "best" for the connection to an access point. It's very good for eliminating the effects of reflections and multipath, but offers no additional gain. When attaching to a single external antenna, the diversity switch should be disabled, bypassed, or ignored.
I don't know what device you're refering to. However, let me point out that exposed center conductor lead length is ultra-critical. Even a few fractions of a mm will make a huge difference in loss. The idea is to use the absolutely shortest length of exposed center conductor from the coax and to use the most extensive and direct ground connection available. Removing the device may also disconnect the circuit board antenna from the transmitter, which is required for an external antenna connection.