Hi Christopher,
1) In theory yes, I believe that's what you need to do, plus swtiching between input and output. In practice I'm not sure it's going to work since the pin, in "SPI mode", is a driven output which then connects to whatever drives the pin when it's in "UART mode". Ie there's a potential short circuit happening there. Perhaps putting a resistor between the pin and the device connected to it "in UART mode" is enough.

2) No. The on the 4550 the UART pins are RC6 and RC7 and can't be moved. If you can use SEROUT/SERIN you can use (almost) any pin you like.

3) You already know my view on this ;-)

4) Hard to say without looking at the datasheet for the UART chip. Some have 5V tolerant I/O some don't.

/Henrik.