For OneShot using Sparky 1.1 and the SN20, you don't have to configure anything on the ESC, but you may want to go in and tinker with the ESC firmware settings to enable dampened light. Not absolutely necessary though. That can't currently be done from the GCS software, and you would need separate software (and hardware) to flash the ESC. Once again, not really necessary.
The user guide for OneShot is still a little confusing, but you can check out https://github.com/TauLabs/TauLabs/wiki/User-Guide:-OneShot125
Specifically, you will need to set the update rate to SyncPWM, and the timer to 12mhz. Also change the pulse duration on all the channels to min:125 max:250.
For question 3, it gets a little more complex. First of all, you will need something to sense battery voltage, deliver that to Sparky, and then deliver that to the D4R-II to send to the Taranis.
You can do this the cheap and easy way by ignoring Sparky and just connecting a FrSky FBVS-01 between the battery and the telemetry port on the side of your D4R-II, but while you will get that info delivered to your Taranis, you won't have it in any other Sparky telemetry.
You can also use the FBVS-01 to connect from your battery to one of the ADC connections on Sparky. To do this you will have to set the OutPort in Hardware Settings to include some ADC inputs, and just wire the output from the FBVS-01 into one of them. You'll then have to go into modules, activate the battery sensor, and configure it with the proper ratios. I can't remember the correct ratio for the FBVS-01 off the top of my head, but it's really easy. Just check the voltage on your battery with a meter and adjust the ratio up or down until it is reporting the correct voltage.
You can also do this with power distribution boards that have built-in voltage and current sensors as well. The APM type power distribution boards generally use a ratio of 100mv to 1v for voltage. They're more expensive than a FBVS-01, but I like the convenience of having it all stuffed together on the PDB.
Finally, getting telemetry from Sparky to your D4R-II is relatively easy, but it takes up either your main or flex port. Run a wire from the TX pin on your port into the green wire that connects to the side of your D4R-II. Go into modules and activate FrSky S.Port Telemetry, and in the Hardware page set the port that you are using to FrSky S.Port Telemetry.
RSSI is a different story entirely. I've been screwing around with the D4R-II recently because of its RSSI output, but was disappointed to find that they use a really screwball high PWM frequency that causes Sparky to freeze up in an interrupt storm. I haven't figured a solution to this one out yet myself.