(1) First of all I will try to explain why what Rav Papa says (top 22a), that the Ganav mentioned in the Beraisa (bottom 21b) is in fact a Gazlan, is not a forced answer. This can be understood through what the Gemara says (BAVA KAMA bottom 57a) that there is an opinion that an armed robber is considered a Ganav, because he hides away from people. RAMBAM (HILCHOS GENEIVA 1:3) in fact Paskens that an armed robber is a Ganav. Someone who steals in public where everyone can see is called a Gazlan, whilst an armed robber is a Ganav even though the owners know that he is taking from them. MAGID MISHNEH explains that the reference is to someone who does not grab money from people by force. Rather he comes to steal in the hope that the owners will not notice. However he carries weapons in case the owners should notice him. They will be afraid of him because of his weapons and will let him take their money without resisting.

Therefore it is not forced to say that the Ganav of the Beraisa is actually a Gazlan, because he is someone using violent weapons, but is nevertheless trying to steal secretly.

(2) I think that the final Teyuvta on Rava can also be understood logically. RASHI 22b DH VE'ISURA writes that there is a Hekesh between Heter and Isur. Just like it is always permitted if it is lost from everyone even though it has a Siman, so too it is always prohibited if it is available to everyone whether or not it has a Siman.

A Hekeish is a strong principle often used in the Gemara. Here it compares two things mentioned in the same verse. In GITIN 41b there is an opinion that a Hekeish is even more powerful than a Gezeirah Shavah.

D. Bloom