Outstanding work and works really well. However the bbcode tags are quite dangerous
I wasn't sure about a numeric value so assumed you meant [preview]
this is the txt we wish to have as our teaser[/preview]
and this isn't.
Submitting that deleted
this is the text we wish to have in our teaser and just leaves and
this isn't in both the body and the teaser (so effectively deleted the text within the bbcode that you can't get back

).
Now if you use it as it should be [preview]28[/preview] so it only shows the first 28 words thats fine and it works well. However the danger is using it somewhere in the body of your text without closing the bbcode and deleting all text within it

BBcode, by its nature, usually encapsules something and i think we'll all assume to encapsule the text we wish to use as a teaser.
think a better solution would be to approach this (if possible) as i tried on my first attempt. I.e. have the bbcode open where you wish to start to new teaser (cancelling the default for that article) and then closing it where you wish it to end. Then thats the new teaser (much easier than trying to work out how many words your new teaser needs). What do you think ?
Everything else appears to work perfectly, including paths, comment box size and even that pesky white line in the username

Outstanding.