I'm not sure regarding the listed stock question, but with respect to the question about entering an order on the NASDAQ, if you don't have a Level II feed, your best bet would be to have TWS display the Best Execution quote (which shows the inside quotes regardless of who is quoting it), as well as ISLD and maybe REDI. If you see that the inside ISLD or REDI quote is the same or near the inside of Best Execution, just fire in your order on the ISLD or REDI routing. If ISLD or REDI quotes are a little further away than the best inside quotes on BEST, then you have to decide whether to give up the spread and get filled right away using ISLD or REDI, or take your chances with Best Execution, which may be routing to a market maker.
Ideally, if you get a Level II feed, you wouldn't need to worry as much, since you'd be able to see whether or not it was a market maker or ECN at the inside, and therefore could easily determine whether Best Execution will work well. If it's an ECN at the inside, go ahead and use Best Execution. If not, I'd stay away.