I believe it was in Bruce's second book that he mentions being visiting by Monroe and a few other deceased people that worked at TMI a few times while he was in F1. It was also mentioned that the woman called Rebecca was visited by her grandmother, which led to the "skunk escapade," as I like to call it.

If those who reside in Focus 27 can "visit" others residing in Focus 1, does that mean they are unlimited in where they can go as far as F1 is concerned? After I die, could I drop by Earth again on occasion and see the Great Wall of China if I wanted, without going through the hassle of travel fees?

You'd probably think of more interesting things to do, but if you wanted to see the Great Wall of China that badly then yeah, you could!   :)

Bruce wrote about Monroe having the ability to be in several places at once.   I think Bruce had an experience like this as well.  I'd say, whatever you are thinking about and want to do, then you can do it.