If Greatjon Umber was excluded by stealth can he jump in as defender in the same challenge?

I think yes, because card ability always has priority against rules.

Well...

Cards do take precedence over the rules, but only when they directly contradict. If Greatjon Umber is bypassed with Stealth, the effect from Stealth is that he may not defend. Greatjon's ability allows him to defend without be declared as a defender. So this isn't a direct contradiction between rules and card effect - it is a restriction that Greatjon has to deal with (or, more to the point, is not able to deal with).

If Stealth said that a character may not be declared as a defender, Greatjon could still jump in after being Stealthed because he was not declared as a defender. But since Stealth says that the character may not defend, period, the Stealth effect (which is really a card effect, not a rule of the game, anyway), having been applied first, makes it so that Greatjon is not able to use his ability.

Now, there is a matter of timing here, though, that really lets Greatjon win over Stealth if you want him to. As described above, once Greatjon is bypassed by Stealth, he may not defend and his ability does him no good. However, remember that you are allowed to use his "Challenges" ability during the Player Action Window between when attackers are declared and the one where stealth is assigned/defenders are declared. So you actually have a chance to jump him in, as a defender, before Stealth is assigned in the first place.

A bit of advanced timing and wording interpretation, but there it is.

If the Greatjon (S10) uses his ability to participate as an attacker, does he interfere with Daenerys's (T106) passive effect ( "if you win a challenge in which Daenerys Targaryen attacked alone..." ) if she was declared as the only attacker originally? It explicitly says he is an "attacker" so you could argue that he does interfere. But then you could counter argue that Daenerys did indeed "attack alone", and later on Greatjon Umber just so happened to appear on the battlefield as an ally.

My guess: he does interfere. It's good to be sure though.

Presumably whatever the ruling for the Greatjon would equally apply to the reliable Walder Frey also?

"Attacked alone" is not determined at the time attackers are declared. It is determined when the challenge resolves. So if you declare Dany as a single attacking character, but there is more than one attacker (because of a card effect like Greatjon's or Walder Frey's) when the challenge resolves, Dany did not attack alone.

There is a section in the rules and the FAQ that talks about attacking and defending characters that are removed from a challenge not being considered as participating characters, or to have participated in a challenge at all. So this "attacked alone" idea being determined when the challenge resolves (rather than when attackers/defenders are declared) is consistent everything else we have.

That was fast ktom! It's almost real-time responses. Much obliged, as ever.