1) Yes, to stabilise hardness and acidity, give the filter bacteria carbonates (which they need to grow), and to replenish other trace elements the fish need. You can either mix with tap water at a suitable ratio (probably about 50:50, but you'd need to experiment), or you can use re-min powder. I personally prefer the latter, as it gives the cleanest possible water (and you don't have to dechlorinate, adding chemicals un-necessarily)

2) Yes, hugely - but if you re-mineralise to the correct level (anything above a kH of 3 degrees) you should be pretty stable. Below kH 3, things can be pretty unstable, but you don't need to be that low for SA cichlids.

(As an aside, 1 degree of hardness is roughly 18ppm, so you kH is roughly 7, and your target would be ~60)

3) Not to a serious extent, as long as you do it slowly - you may find that you have to slightly increase the amount of re-min powder or the ratio of tap:RO water to maintain it, but it will be a gradual change which you should be easily able to keep on top of.

4) What are you missing? Not a lot: lots of bogwood and a kH of ~3-4 should give you a nice pH in the mid 6's. RO sounds scary, but all you're basically doing is removing the minerals from your water, then re-mineralising back to a level you'd prefer.

My advice: get a good RO unit and use re-min powder. Get an RO/DI unit with 3 stage pre-filter (sediment, carbon and "chlorplus" or similar) and you can use the pre-filter as an HMA filter too, rather than using tap water.

I'd also suggest getting a water butt and pump, and pre-mixing your water before adding it to the tank. It makes it much easier to mix the water to the correct level, and monitor the levels.

I found moving to RO/DI took me a couple of hours setup time, a couple of hours working out what kind of ratio of powder:water gave me good results, and then about 10 minutes extra per water change.... but I also saved 20 minutes per change by using a water butt and pump rather than lugging buckets around :p