I'll have to leave it to others regarding the open houses, but as far as your visit goes, I would highly recommend that you call and schedule a Lifestyle Preview. You will get a lot more information that way, and you will have a Gate Pass to get into the various villages (using the golf cart that they give you during your stay). Unlike other urban or suburban areas, the best way to get around in TV is the golf cart.
Regarding the trolley... it's not a form of transportation. Think of it more as (about an hour long) tour of TV with a guide. It just gives you a general overall understanding of the area during that time. You schedule your ride at the Lake Sumter Landing Sales Office. This was done for us by our sales person while on the Lifestyle Preview. There was no sales pressure at all... just good information.
Once again, others will be able to address the Open Houses question better than I, as we primarily looked at new construction. We are planning to return for a second visit soon.
A few days (or even a week) is not enough time to take it all in and make any sort of decision.