It would really be helpful to be able to play exactly the clip of the video that you just tagged with the start and end tags. This way you would be able to know if the video that you are going to extract starts and stops exactly as you wish it to.
I don't know if it is possible to do so in the current version of avidemux - when I used the view function it would not stop at the end tag of the clip.
view/pre-view/play from (marked) A to B is not present/available in 2.6. Suggest you change the Subject and make it an "Feature Request: ". You never know, developers could implement this request.