In a sentence: I went to New York, too. q1: What was presupposed the thought of the speaker? 1) I went to Boston. 2) He went to New York. a1: What do you mean about 1. and 2 ? This is a speaker thought sequence ? Speaker 2 states this “I went to New York, too”. That implies that at some point in the conversation before, speaker 1 stated that he has visited New York, and speaker 2 stated that they have been to New York as well. So its more like 1) I went to New York 2) I went to New York, too. q2: Is this "too" replaceable with "as well?" a2: You mean the “too” in “I went to New York, too.” Yes, you are correct. This has the same meaning as saying “I went to New York, as well.”

Yes. You can use 'also'