Yes, you've pretty much got it there. I'll just clarify and expand upon a few things. First off, whenever talking about your carbs, you should state the year and type of carbs you're working with. I can pretty safely deduce from your mention of the mix screw location that you have BS34s, but many others wouldn't know this. OK, on to the critique .....
Step 1 - When I bench sync, I don't use a feeler gauge or spacer under the butterfly plate. Instead, I simply observe the cracks of light around the plate, mainly at the top and bottom. Holding the carb set up to a good light and looking in at the plates from the engine side, I back the idle speed screw on the left carb out until it doesn't touch the cable arm at all. If in sync, both plates will be completely closed and show no light (or very little). If one is open and the other closed, the carbs are out of sync. Adjust the sync screw on the linkage between the carbs until both plates are closed. Now turn the idle speed screw back in until it touches the cable arm, then give it 1 to 2 more turns in. That will crack both plates open a bit and give an initial idle speed for 1st start-up. Once you start the bike, the idle can be dialed in better, to 1200 once warm.
Step 2 - If you've done a proper bench sync, the carb sync will be close so you can do an idle speed AND mixture screw setting now.
Step 3 - Yes, now do the sync with gauges. If the idle speed changes due to your sync adjustments, re-set it.
Step 4 - Yes, re-visit mix screw settings and tweak. If this results in a higher idle speed on either carb, you may need to tweak the idle speed adjustment and the sync setting slightly.
As you can see, fine tuning isn't a "cut and dry", one step at a time procedure. You will often need to re-visit some of the earlier steps and tweak things. That's the "fine" part of "fine tuning".
And to answer your two questions, adjusting the idle speed screw will not effect sync. It will simply raise or lower the idle speed of both carbs together because they are connected through the linkage. The idle speed adjustment screw moves the whole linkage so it effects both carbs. The sync screw only acts on the right carb to allow matching it's speed to the left.
Adjusting the mix screw can effect the idle speed and the plug color. If you're off too far one way or the other from what we call the "sweet spot" (the ideal setting), the idle will be rougher and slower. The plug color can also go darker or lighter depending on if you're set too rich or too lean. You should try to achieve fastest, smoothest idle when setting the screws.
