We don't know that Lulach was feeble-minded: history was written by Canmore's descendants. He was unlucky, certainly, but we can't say for certain whether he genuinely was simple or not. I have a sneaking suspicion that if he were simple, someone else would have been chosen for the job by Macbeth himself (he wasn't even Macbeth's biological son).
Supposing Lulach turns the tables and kills Canmore, it doesn't leave him completely clear - he would still have to face Malcolm's brother, Donald Ban. Malcolm was sponsored by the English, and without him as their figurehead, they would *maybe* focus their attention on sponsoring his brother. But then, Donald wasn't nearly as receptive to English ideas of cultural superiority as his brother was, and when he eventually did become king he made a lot of enemies by throwing out the colonists who had come with his sister-in-law Margaret. Without English aid to Donald, Lulach should have no real problem holding Scotland. After all, his step-father didn't, and reigned for 17 years, which is pretty much a record for any Dark Age king of Scots.
There certainly wouldn't be a Pictish resurgence (the Picts were long gone by this point), but the House of Moray would continue to dominate a Scotland with a Gaelic speaking court. There will be no St Margaret and, as a result, no poncy, southern, wine-drinking Anglo-Saxon refugees to bring European feudalism to Scotland. The centre of power won't gravitate to Edinburgh, but probably somewhere more central - Stirling or Perth, nearer the Moray power base. If the Scots kings ever get round to having a formal capital, that is.
At the same time, you can expect some kind of Norman infiltration to begin in earnest. Macbeth is known to have deployed Norman knights, so it's likely that a few hardy adventurers will set up shop north of the border, though nowhere near the scale that they did in real life.
Lulach probably won't consider Margaret for marriage, as he was already married at his time of death and had at least one son called Malesnecta. Whether some kind of subsequent union between the House of Moray and the House of Wessex might take place, I don't know, but I'm inclined to say it won't. The circumstances that led to Margaret being in Scotland in the first place were very random, and dependent on luck: kill Canmore, and I'd say they butterfly away.
Although I would expect the English kings to continue sponsoring pretenders, eventually it might become politically expedient to come to some kind of agreement with the Moray kings. That leaves the Norwegian-dominated Northern and Western Isles to deal with, and the corresponding low-level thuggery associated with having Scandinavians occupying adjoining territories to your own, but the Moray kings had been dealing with that for centuries. Malcolm II had drawn the Orkney jarls into his orbit in the preceding century, so we can expect some kind of loose relationship to continue developing between the Northern Isles and the mainland.