In a phrase of -고 싶다, the particle '이/가' and '을/를' are interchangeable each other. The meaning not depends on '이/가' or '을/를', but depends on the context. Here '이/가' is used not as a subjective particle but as an auxiliary particle.

No. the meaning changes depending on the context. 이/가 을/를 changes depending on the noun (who) 보고 싶다 therefore means both to miss and want to meet sb.

no. the meaning changes depending on the context. 이/가 을/를 changes depending on the noun (who)