Several Things.
1. Counter-Intelligence allows the use of some things that aren’t legal with criminal investigation. That is a huge factor in what they did with the FISA database.
2. Comey lied before Congress.
Comey has long checked all the “eligibility boxes” for a DNC office somewhere.
No, you have it backwards. They used FARA so they could go to the FISA court and get a warrant under title VII of the foreign intelligence act instead of Title I. Under title I, Carter Paige would have been considered as helping a foreign agent and they could listen in on his phone calls only when he was speaking with a non US citizen. Under Title VII he is considered a foreign agent and they could listen in on any of his phone calls regardless of who he was talking to. They could also listen in on the phone calls of people he had talked to, and in some cases the people those people talked to. This is how they were able to listen in on Trump and his campaign staff.
Trump knew this and it is why he said they tapped his phone. They did under the warrant they got for Carter Paige as a Soviet spy.