Enqueue: add it to the queue (for playback); or (in other words): add to playlist.
Enqueue selection: add more (selected) files to the queue (=playlist).
Enqueue Shell Extension: a rightclick on a file offers i.a. enqueue-option.
Where is it stated that new playlist has an advantage over old playlist?
You can use either of them depending on your personal preference.
If I remember correctly new playlist came with version 2.11.940.
(ML was already introduced with version 1.38.828)
It has a more modern look, more in line with ML and also has the tags (at the top) like those at the right hand side of version 2.x base skin (video, dvd, audio, radio, tv).
I've no idea starting from which version BS.Player supported flv-playback. (Why do you want to know?). I only know that broken FLV
streaming support was fixed in version 2.33.977. I think BS.Player has been able to play flv files starting from version 1.x (in version 0.86 users couldn't add "associated extensions" yet) resp. from the moment there was an appropriate splitter+codec/filter available.