Pending money to unlock the Silver pack early one won't help, because it's recommended to fully upgrade commons first, yeah. You want to max all of them or almost all of them. A lot the uncommons are remarkably non-essential, more so than the commons.
Btw, click "Herd of Boars", "Ritual Sacrifice" and "Orphan Gang" whenever they come up. They comprise the best budget deck in the game, although you won't be able to play it at all before you're finished upgrading the Uncommon boars. The deck can use some other stuff like Rite of Combat, Lepers, or Scrivener, but those are just seasonings and aren't as pivotal to it.
For the sake of full disclosure, "not yet finished" would technically be a misleading representation of the campaign. They aren't really working on it. They started focusing on draft, and then new cards instead. I don't know how frequently they are adding new content at all, it's been three weeks since I got back into it and I haven't seen anything new, but I like the stuff that's already there.
The medium difficulty level of the adventures is pretty hard. It can take like 8 tries to beat some of them. I will tell you that when I first unlocked the medium difficulty I was not ready, and I ignored those campaign missions for a good long time.
Reportedly the hard difficulty level on the campaigns is absurd, and requires a totally complete collection, skill, and luck.
You can't upgrade cards within a game, but when you have upgraded a card you can use the improved version of it and the weaker version it both in your deck. They might have been touching on that in the tutorial. They made the tutorial a loooooong time ago so it's likely just wrong though.
Loan is a really bad card before you upgrade it. Upgrading a card reduces how much gold it costs you to buy it. For a card whose main purpose is fulfilling a certain role, like "provides you health" or "destroys an ally", that can be unimportant. For a card whose main purpose is to give you gold -immediately-, the -immediate- gold cost is really relevant. I don't think I've ever seen anyone run the 4 cost or 5 cost versions of Loan in a multiplayer deck, only the 3 cost version of Loan, fully upgraded.
The AI probably just got lucky and redrew the Fiefs they bought a ton. Or maybe they did the cute "debt collectors don't know about my offshore Magic accounts" thing, which is difficult to pull off consistently.