I think given the age of the GoH, Dave really should have checked with Linda before saying he'd go. As an 11 year old, Pam likely doesn't precisely understand the dynamic between her parents (it's possible that she sees them getting along well enough when they interact, but doesn't realize how hard it would be for her mother to have her ex-husband in her home). She also isn't old enough to invite people to her house on just her say-so, in addition to the fact that while she might be the GoH, she wasn't the hostess. In that situation, she gets to have input and can make requests related to the guest list, but it's Linda's house and Linda is the hostess, so she gets final say (moreso than she would if Pam weren't still a minor child in her care).
I do think it would have been gracious for Linda to allow Dave to attend the party, had he asked to, because Pam obviously wanted him there. But at the same time, I completely understand why she would be so annoyed that he just showed up. Pam might not understand the dynamic between divorced parents, but Dave sure as heck ought to. Linda would have been well within politeness to turn him away at the door, but I imagine that would have left Pam rather upset, so I'm not surprised she didn't do it.
It would have been rude for anyone to wander the house when they weren't invited in (since the location of the party really was the yard), but it's particularly awkward that Dave did it. I don't think he was any ruder than anyone else would have been, but I do think his rudeness was far more likely to feel sort of unforgivable to Linda.
I don't think there's anything else Linda could have done without disappointing Pam right before her party. I do hope that she had a talk with Pam about not adding people to the guest list without checking with her host in advance, because even as the GoH, she still can't invite people without telling the person who is making the preparations for the party.