You're never going to have an indefinate run of Scottish losses, just as you're not going to have an infinite run of victories - as I keep pointing out, the English can't maintain a large army on Scottish soil, and if they do not, then the Scots have the advantage of local superiority.
And you overestimate the difficulty of doing so, and the ability of the Scots to keep resisting. Burning fields again and again, losing men again and again, is going to weaken Scotland.
Sooner or later the English are going to lose interest, as they did. In theory, do the English have the resources to subjucate Scotland? Maybe. But the more the English spend on maintaining Scotland, the shakier their positions in France, Wales and Ireland becomes, not to mention the political situation at home.
Sooner or latter the Scots are going to lose hope, if their wins keep being opposed by English wins and Edward II strengthens his hold on the country.
Obviously the English can't continue indefinitely, but what are they facing that means that what Edward I did is infeasible for his son, assuming junior is up to the task of pressuring Scotland?
Food is wealth. As most of Scotland is not arable, most of its agricultural wealth is composed of herds of cattle, goats and sheep - in other words, moveable food. The Scots destroyed what they couldn't take with them - and in any case, it would be a mistake to think that they burnt the entire country down. They only destroyed what was in the English line of advance. Since the English very rarely got beyond the Forth, the rich agricultural lands of Fife and Angus were hardly ever touched.
And if the English get to and beyond the Forth, then what?
If the English are able to hold the area short of the Forth, then what?
I don't know enough about Welsh castles to judge. Certainly Stirling, Edinburgh and Roxburgh castles were made of stone, and the first two were in very strong defensive positions. In any case, castle-building is a very expensive ploy and you need to have more control over the Scottish countryside than the English ever had.
More than they ever had when Bruce was in hiding on some island and inspiring the spider story?
An English army invading Scotland musters at York, not Edinburgh, Newcastle or anywhere like that, because most of England's population and baronage are based in the South. Possession of Lothian is almost irrelevant, and was never entirely secure anyway.
Very relevant for supply lines.
The supply lines needed to reinforce an army travelling through the Highlands are indefensible and very long, not to mention the cost of supplying such a force - logistics matter just as much to a medieval army as a modern one, and are actually much more difficult to maintain. Hence why very few medieval campaigns are of a length much longer than a summer, and hence why the English will find it very difficult to conquer Scotland - because they can't sustain a campaign year after year.
As for sustaining a campaign year after year: So what do you consider the campaigns Edward I launched?
Why do you assume that the English capacity, or more accurately, will to keep coming back at the Scots is much greater than Scottish capacity to resist? The English were only ever able to send large armies north of the border a couple of times in a decade. The more attention Edward II devotes to Scotland, the more restive his barons back home get. Every major Scottish campaign requires extra taxes. If they're not getting any return for that, why are they going to tolerate continual high level warfare in a poor land? They would bankrupt themselves eventually.
Because of the historical performance of Edward I vs. the Scots. You know, the Edward I that had suppressed (temporally) the rebellion by 1304? He didn't do that by bribing them to shut up long enough for him to claim victory and get out of there while the getting was good.
As for twice a decade:
Edward's initial invasion, deposing John Bailiol: 1296
Stirling Bridge: 1297
Falkirk: 1298
Other campaigns as well, but that ought to be enough to indicate that the English are doing much more than "twice a decade".
The Scots showed themselves able to keep going despite the loss of their leaders in both the First and Second Wars of Independence. At the beginning of the first, most of Scotland's natural leaders were locked away in England or serving with Edward in France. At the beginning of the second, almost all of the country's leaders were killed at Dupplin Muir and Halidon Hill - but their heirs, who were in some cases no more than teenagers, kept up the struggle. The notion that the English are going to be able to capture, kill or incapacitate everyone who is able of leading a rebellion doesn't stand up to the record of history.
The notion that the Scots are going to be able to keep coming up with more leaders doesn't stand up to reason, either. If the men leading the schiltrons at Bannockburn are killed, how many men are there to replace them?
Referring to that because those are the men who will be lost in successful campaigns by Edward II.
At the end of the day, what does Edward II being competent change? He's able to win battles. So what? Most of Scotland was won for Robert Bruce without any actions larger than a few hundred men. In fact the Scottish strategy was to avoid pitched battles. He still has the same difficulty keeping his armies in the field that he, his father and his son had, he can't afford a prolonged, grinding campaign - and even if he wanted to, his barons wouldn't let him.
He's able to win battles, retake areas the Scots temporally occupied, and send forces to control Scotland. The Scots avoiding pitched battles makes things more difficult, but not impossible.
The problem is that the Scots are fighting on home ground and the English aren't. The English lack the political will and the military ability in terms of logistics and mobilisation of manpower to actually conquer Scotland anything other than temporarily, unless the Scots just fold - which history suggests they won't do.
Because the Scots will resist to the death. They will continue fighting no matter how many nobles are killed. Kill the Bruces off? No problem. The Douglases? No problem. Stewarts? No problem. Etc, etc.
Are we really supposed to believe that the Scots are all fanatically dedicated to independence to the point that no matter how many defeats they suffer or how many leaders are killed that it won't matter?
I don't think its being suggested that the Scots just fold, just that their ability to keep going is finite.
A Scottish army so deep in the Highlands as to be hard to reach is also a Scottish army so deep in the Highlands as to not be much use in determining the fate of the Lowlands. Using "army" in the sense of "armed force able to cause trouble" - whether its a pitched battle force or a guerrilla force.
If this is true:
http://www.scottishhistory.com/articles/independence/summary/wars_part1_page3.html , Edward invading between 1307-1314 (as in, at some point between, not every year - though he can probably launch more than one invasion) would be a problem for Bruce's attempts to tear down castles and beat opponents.
Referring to the third to last paragraph - if Bruce's position is easier without that, logically his position would be harder with that.
Also, I have to wonder what kind of anti-guerrilla tactics are an option in the Middle Ages. Destroying villages is going to make angry Scots...oh wait the Scots already are angry.
This being said, I do think it seems like a poor use of, in a word, resources - men, money, good will, and royal time (time spent on Scottish campaigns is time not available for any other projects Edward II wants to see accomplished, just as manpower, money, and good will are used that could be used elsewhere).
Pity that there's no good alternate candidate to Bruce. Looking at this not so much as a puppet as a way to bog Bruce down establishing his claim.