The rules for joust say that up to 1 defenders can be declared when character attacks alone (or something like that). My question is: can you use effects like Catelyn's (LoW) and declare a defender, or does Catelyn's effect count as declaring a defender?

You can still use Cat on top of a decalred defender. Declaring and jumping in are two diff things.

The important distinction here is that any effect that says something like "have this character participate in a challenge as the defender" is NOT "declaring" as one. Declaring defenders is a unique framework action that occurs after attackers are declared (and following the Player Action window).

There is also an important distinction that jumped in characters are not "declared" defenders, but they are "participating" defenders.

What happens if you attach Tourney Lance to the wildling card Orell the Eagle says that "opponents must declare at least 2 defenders in order to defend the challenge?"

Without the attachment, they have to declare 2+ defenders in order to defend the challenge. However, even if they don't declare defenders they can still use jumping abilities to add participants into the challenge.

With the attachment, since Joust only allows 1 defender to be delcared, and his ability only allows 2+ defenders to be declared his attacks will always go without defenders being declared. But other affects can still allow characters to join as participants.