UART: two transmitters, single slave
To answer your questions:


Yes adding a 1k resistor in series would work. They ATMEGA output pins can put out up to 40mA without being damaged, so if there is a conflicting 3.3v transmitter on the line, a maximum of 3.3mA would flow. However, see below:
Yes it would be in high-impedance mode, so you can test the BT slave using the FTDI connector before you program the ATMEGA.


Given point 2, there would be no conflicting transmitters, assuming you only connect FTDI before the ATMEGA is programmed. Therefore, the 1k resistor isn't actually necessary. Besides, I don't think the FTDI would be capable of successfully transmitting over the ATMEGA even with a 1k resistor between the ATMEGA and the BT slave. It depends on your programmer.

As an alternative, I would use a multiplexer to switch between transmitters in a controlled way. This could be done with just a few basic logic gates, and would make your design more resilient and flexible.