remember that all ski maintenence techniques are derived from racing. And in fact, most of the things like super edge polish have been developed to make a speed ski the absolutely fastest it can be.
1) even for race skis, some of what we treat as gospel is not necessary for technical events- that is, there are so many factors involved in a slalom run, that the perfect polish on edges and perfect base structure are not nearly as important as they are for speed skis- it is blasphemy to say it, but I might go so far as to say that in many instances they are not critical. The skis need to be properly sharp and pretty slippery- then it is really about the line and execution in SL.
2) why tip to tail? every action we do to the base a a ski leaves little tentacles behind it (i.e. small hairs of base material and wax, or fuzzies- call them what you will- you can see them with a reasonable magnifying glass and have no doubt seen them in a macro form on burned bases). As we use successively softer and finer tools on the base we are intended to be removing the larger tentacles and they get smaller and smaller. But they do exist. Acknowledging that they do exist, we want them to lay down from tip to tail so that when we ski on them they create the least possible drag. Think of them like the hair on a pair of climbing skins. In one direction climbing skins do not allow your ski to glide (i.e. they stick so you can climp up hill), in the other direction the hair on skins folds down and you can glide.
3) brushing sideways, backwards etc- I wouldn't do it on an entire ski, but if you are tuning anything other than SG and downhill skis don't worry about doing it locally do deal with a wax glob. You will not hurt the base structure unless you go at it pretty aggressively with a steel brush. That said, the approach we use on speed skis is to pick up the scraper again and scrape locally (tip to tail) to treat the glob and then brush some more
last word- DON'T worry - just the TLC of a wax and tune is 95% of the battle- many folks are way to rigid about following all the rules. Your going locally across the ski and then tip to tail over will be imperceptable. A deep dark secret, I have prepped SG skis and (horrors of horrors) and run the wax iron both tip to tail and tail to tip- please do not let the tuning gods know-