I've built a Softrock RXTX kit (full schematic: pdf gif, see lower-left corner). It has one connector for one antenna used for both receive and transmit, but I don't fully understand (likely due to my inexperience with RF design) what all the parts do. The full schematic is a mess, so I've done my best to transcribe the most relevant parts:

So, I understand that when \$\overline{PTT}\$ is low, then Q10 is off, which isolates the receiver. Q11 is on: why?

When \$\overline{PTT}\$ is high, Q11 is off, which is good, since I don't suppose we want to ground the receiver's input. How do we know that Q10 will be on, given that its source is floating?

What does C27 accomplish?

Say I had an RF amplifier and I wanted to add a similar circuit to bypass it when receiving. Would I have to worry about disturbing the impedance matching?

What should I consider when selecting the transistors to use?