Registration-Another State

no need to register in Andhra. Just ensure that you are issuing bill of supply instead of tax invoice in Andhra. 

you have to register in andhra pradesh

I think exemption isn't the criteria to be registered or not . Though you are crossing the threshold limit you should have to be registered yourself , only if the goods are non gst then you can stay away from the registration. 

please refer: https://cbic-gst.gov.in/faq.html

no. you can take voluntary registration at nil turnover too. The query is whether registration is required or not, not whether he can take or not.