Note that on the simple board, the TX/RX lines are only available on the 6-pin header. So if you need those, you'll have to tap off of that header. If you need to reprogram the Lilypad, you're going to need that header as well.
As for your question of whether an FTDI is needed: only when programming the sketch into the Lilypad. After that it's no longer needed. And rather than using the coincell holder with the main board, I would opt for the LiPower module: LilyPad LiPower - DEV-11260 - SparkFun Electronics (this is not needed if you use the simple board as it has a build-in connection and charger.)
Another possibility is that you create/design your own board, with the specific pins you need. Been there, done that.