Italian Speakers, can you check this over?

I've read it over and it's good effort, but there are a few errors, or weak spots.

One thing that you have done repeatedly is to use the presente progressivo (stare facendo qualcosa) incorrectly. The present progressive is used to indicate something that was happening EXACTLY at a certain moment. It is used FAR less then in American English.

You are at home taking a nap, and the phone rings, and the person asks what you're doing. Sto (stavo): I am/was sleeping. You are saying what you were (in the act of) doing JUST THEN.

In many (most) of your sentences that use this, you should use ordinary present instead (or whatever tense applies).

James: “Perché tu siamo ridere?” : What?? Why/you/we are/to laugh Better: Perchè (tu) ridi?

Maria e il tassista ridono. Maria ed il tassista....(e>ed before a vowel sound). Il mio fratello ed io.

Tassista: “Perché sei in Italia?” The taxista is using the informal with a customer. Maybe to a young person,

....but not to an adult.

James: “Lei è.”. In your (English-speaking) mind, you're hearing the taxista's questoin (Chi si sposa) as "Who IS getting married?" and reply "She is". An Italian would more likely respond: "È lei" (it's her)

James: "E 'questo ok?” In short questions like this, inversion is often not used: Questo è OK?

....better still: Può/possiamo pagare con dolari? Lei acceta i dolari? or something similar.

Although written correctly, the dialog about paying the taxi driver is unclear to me. Does the taxista return the dollars to James and then accept € from Maria? This could easily be clarified.

Despite these issues, I have to say that you have a very good grasp of the language, and the dialog is well thought out and logical.