Well there's also a reason why Comyn is a name more or less forgotten in Buchan today, and that's because they changed it to 'Cumming'.

The Herschip of Buchan was probably a necessity given the immovable opposition of Comyn/Balliol kindred to Bruce - he needed to break their powerbase permanently. Whether the removal of the Comyns was overall positive is perhaps another matter - it removed a stabilising influence in the north of Scotland and created a power vaccuum - but from the viewpoint of the Bruce party it was essential in order that they could unite the country behind the Bruce dynasty. In any case Robert made provision for the north of the country after the downfall of the Comyns, it was only later on in the fourteenth century that this really fell apart. That's also why I disagree with you about the murder of John Comyn. The presence of an overmighty rival to Bruce on the patriotic side would have been intolerable, and Robert I would have had to take action to neutralise the Comyn threat eventually. Besides there was no guarantee that the Comyns would remain upon Bruce's side.
In any case there are doubts over Bruce's motives for killing the Red Comyn - the idea that it was premeditated is only one theory, there is a possibility that Comyn had already betrayed Bruce to Edward I, another is that Bruce simply lost his temper.
The Comyns were not brought into Bruce's peace because they showed no desire to do so. Of the rest of Bruce's enemies, even those who had been consistently opposed to his claim to the throne - such as the Earls of Fife, Dunbar, Mar and Ross - were accepted back at court with their lands intact, in the cases of Fife and Mar over a decade after Bannockburn. Maybe you could argue that Bruce needed their support and thus had no other choice, and you'd probably be right - but the fact that he was willing to recognise this and act upon it still makes him a pretty good ruler by any standard.
There's no doubt that Balliol inherited a bad situation, but as you say yourself he was not up to the job at all. In 1294 he was deprived of power by the nobility and clergy - by his own Balliol and Comyn kinsmen - and effectively made a figurehead for the council that approved an alliance with France and shook off allegiance to the English. I'd say this makes him a fairly weak and rather pitiful figure, and while he was vilified by Bruce propaganda in the next century, that he was effectively deposed by his nobility speaks volumes about his ability as a king.
I'm perfectly willing to concede that the actions of the Bruce family in the early stages of the Wars of Independence were hardly patriotic, especially those of Robert I's grandfather, but Robert himself has a reasonable record prior to taking the throne - he didn't oppose Wallace at Falkirk, he wasn't there because he was governing the South West of the country for the 'patriotic' cause. In any case I doubt many contemporaries would particularly have marked him and his family down for switching sides: preserving the interests of your family was seen as your first priority as a nobleman, and national identity was a sketchy concept in the late thirteenth and early fourteenth century anyway, especially if you owned substantial lands in Ulster and England as well as Scotland like Bruce.
The actions of Bruce as King of Scots do mark him down as heroic - look at his two years on the run from Edward I after his seemingly conclusive defeat at Methven, building up the loyalist cause rather than running off to Ireland or Norway and never being seen again; look at his skill in defeating the English through Fabian tactics and a pitched battle in which Edward II had an overwhelming superiority in manpower and weaponry; look at his statesmanship in managing to reunite warring factions and gaining recognition of Scotland as an independent and fully sovereign nation on the European stage (even from the Pope in the end, who granted Bruce the full rights of anointment and coronation at the close of his life, thereby elevating Scotland to the relatively select rank of recognised European kingdoms); look at his efforts after Bannockburn to come to an agreement with England guaranteeing full Scottish independence and permanent peace (btw the fact that Scotland was able to pay Edward III £20,000 sterling, an absolutely staggering amount of money then, within two or three years in the period immediately after Bruce's death in 1329 is a good indicator of how well Bruce governed the country). Heroism is somewhat subjective, but I'd argue that regardless of whether you view the outcome of the Wars of Independence positively or not, Bruce fulfills most of the criteria.