Yes, the airside walkways have always been there and are not going away. The signage is not perfect, but it's adequate. The only time you'd HAVE to exit security (and board the ATS) in to change flights is to get to/from T5.

Time really depends -- T3, in particular, has four long concourses ("arms") and no moving walkways. High-numbered gates in H/K/L will be a long walk away. The CO/NW lounge is at least at E4, not too far within T2.