Depending on which realm I am from, does that affect whether I am allowed to trade in Asgard and open a ThorFX account?

Yes, it does. If you are from any of the below realms, you will not be able to trade with us.

Afghanistan, Botswana, Cuba, Democratic Republic of Congo, Ethiopia, Iran, Iraq, Japan, Lebanon, Libya, Malta, Myanmar (Burma), North Korea, Pakistan, Republic of the Congo, Somalia, Sri Lanka, Sudan, Syria, Trinidad and Tobago, Tunisia, Vietnam, Yemen, Zimbabwe.