It's not a goof that only Thyra's blood would work to help send Ragnar to Valhalla. Thyra is Ragnar's blood sister. Uhtred is not related by blood.
When Uhtred and Brida enter Storri's cave, there is a bearded dragon lizard in a cage. Bearded dragons are endemic to Australia. There is no conceivable circumstance that a species from Australia could be in 9th-century Britain.