OK so
1) The volca has no USB so it will be connected via a MIDI cable, I guess via the Scarlett unless you have a USB-> MIDI cable in one of the computer's USB ports. Volcas only have MIDI in so the MIDI will be going from Live to the Volca and will be an output in Live's MIDI preferences.
2) Whichever port it is connected to, you should have the sync button lit up on that MIDI out, that will put the MIDI clocks in time. As above.
Generally the track button sends/receives notes and controller data, the sync button times up the clocks, and the remote button is for Live's MIDI learn function which is generally only used on inputs. If you only want the sync function you only need to have the sync button lit up.
It is also possible if you use an external instrument to sequence the Volca direct from Live which avoids the whole issue. I find the volca's memory of only 8 sequences to be a bit limiting so I tend to use this method. There is a max for live device called J74 that will let you MIDI control almost all of a Volca's parameters from live, which you can get here.
http://fabriziopoce.com/volcacontrol.html
Alternatively you can set up a drum rack to do it as in this thread
viewtopic.php?f=1&t=222049 (or just use a piano roll clip).
The Max device is probably the easiest to use but has a small cost.
For both those options you will want the track button lit up, the sync is not necessary if you aren't using the Volca's sequencer. If you're doing this and you do decide to leave the sync on you will probably want to clear one of the Volca's memories or it will start playing whatever is in the memory instead of what you have in Live. I generally have the first memory slot blank on my Volca beats and keys for this reason.
Finally the Volca's own sync out/in is just a series of audio pulses on 1/8 notes. I have found that if you record the Volca's sync out signal as audio into Live (just connect a lead from the Volca's sync out into one of your audio ins and record it), use warping to time it up and make sure that the pulses are on 1/8ths, you can then send this as a sync in signal via the Volca's sync in port and it will synchronise the volca. This has the advantage that you can use the Volca internal sequencer (assuming that is what you'd like to do) but it is very easy to go half time/double time etc by using the half time/double time buttons on the audio clip. If you're doing this you don't need to worry about MIDI sync at all as you are using the Volca's sync pulse input which is not MIDI.
Lots of things to play with really.
(mmmhhh.. not good thinking of ableton at work haha) 