You can most certainly go from VA 73 FO to TT Command. In fact it looks as though that may happen in the near future given that their is only about 15 TT FOs before the rest of the VA pilots will have access to command (like what happened with VANZ).
My understanding is also that you do not require NB command to be eligible for WB Command. This was changed in Experience Requirements about 18 months ago. But Bereal is spot on - WB commands are going very very senior (first two pages of GDOJ list) and WB FO positions are going very senior too (a 1000 staff number 73 Capt recently took 330 FO). Also rumour is that the next WB FO positions will all go to 73 captains as a lot are looking at a better lifestyle.
The ATR CAP still looks to be phased out in the next EBA.