Revoking titles in your primary duchy is extremely helpful, and if you do it via plot (intrigue tab, "click to choose a plot," "revoke county of X") you won't take any opinion penalty from your other vassals. Acquiring provinces up to your demesne limit should be an early-game priority for three reasons. First, because when you're a small realm your personal levies are the backbone of your army, second because it will significantly increase your income, and third because having a lot of personal levies will keep factions from revolting against you. As already noted, you can also expand your demesne by taking provinces from the duchy of the Isles. Even once you're at your demesne limit, try to keep acquiring personal control of counties; you can give them to family members, vassals with the content trait, or claimants to titles you want control over, as you prefer.
You may end up losing Cumberland - that's ok. Don't worry about it too much - England is bigger than you right now. But they're only claiming Cumberland because they have a de jure claim; they probably won't follow up by claiming other parts of Scotland. If England declares war on Cumberland, you have two options; call in allies, hire mercenaries, and try to fight back (and they're likely to do it again in ten years even if you win), or let them have it without much of a fight. What you definitely don't want to do is waste your army fighting a war against overwhelming odds - after you're decimated is when factions will revolt and other claimants will attack.
But you don't want to just surrender, because while the English are attacking you get a massive "defending against foreigners" opinion boost with your vassals. So let England take a few years to siege down all the holdings in Cumberland, because while you're "defending" against England is actually a great time to raise vassal levies and attack Norway or the Isles.
Your early game goals are simple. First, consolidate personal power by revoking counties. Second, consolidate realm power by conquering all of de jure Scotland. Third, pick off nearby targets of opportunity to build a power base. Ireland and Wales are weak and divided; the only thing standing between you and those kingdoms is getting claims. Invite/land claimants, marry for claims, or fabricate claims, as needed. Once you have most or all of Ireland and Wales, you should be able to stand up against England, so try to acquire claims and press them when England is weakened by wars or revolts.
One final point - as your realm expands, vassal relations become extremely important. One of the best ways to keep your realm stable is to make sure all your vassals are the same culture/religion. So when you conquer areas outside Scotland, you'll want to do one of two things. First, you can revoke titles (or personally claim them in the first place) and give them to Scottish courtiers. Or, you can arrange for a Scottish guardian for your vassals' heirs.
