Hey it's your thread so you can ask what you want so I'll answer what I know how to
Can some people switch whenever they want?
Yes, some people can. You can learn more about co-operative switching if you search about it on this forum. I know Johnny-Jack wrote about it not long ago at least. But the fact that Leo wrote after me was not something I did on purpose. I'm not strong enough to keep her away even if I wanted to. She decided to pop out to write something, was all.
If the environment is completely open does that mean that for some, all personalities can see ( and think about) whatever the host is viewing?
The inner world is different for everyone so what may be true for one may not for the other. So that depends. In our system some alters tend to keep track of and think about/comment on what is going on when someone else is fronting. One of the kids for example ha no idea what's happening on the outside (the baby) but others can "pop by" when they want to check what's going on. It also depends, for us at least, who's out and such. Like someone can watch alter x and z but not alter y but alter y can see everyone for example. Kinda hard to explain. But to answer the question: in theory, yeah I guess that's possible.
Can the original personality decide to switch main role with one of the alts?
The topic of "original personality" has been discussed many times here and you might find it interesting to search about it. If you mean original personality as in the host (the one who is in the body the most) then yes, they can switch roles and have someone else be out the most.