Chances they make gay marriage legal in all 50 states (under 14th amendment grounds or otherwise)?
Less than 50%. If they affirm a right to same-sex marriage, it will be on Equal Protection grounds. There is a chance they backdoor it by stating that the House lawyers don't have standing to defend DOMA. That would have the practical effect of causing the Federal government to recognize state recognized same-sex marriages without issuing a precedent that says the government has to recognize those marriages and without having to tackle the issue of whether same-sex marriage is a constitutional right.
Zero. In the court's precedents, there are many examples unrelated to marriage where two states have laws covering the same subject but have different standards. The court has consistently said that one state can't force its standard upon another. The court has fairly consistently held the Full Faith and Credit Clause to only apply where two states have essentially the same standard in place.
