One trouble with extracts is that we, the brewer, have no control over the fermentable/non-fermentable mix of sugars, and in fact they are usually somewhat less fermentable than a similar AG batch. No big deal and very easy to compensate for with normal strength brews.
enzymes in the mash work on breaking down the starches and complex(non-fermentable) sugars into simpler (fermentable) sugars.
If you did not get enough enzyme action there is nothing that will allow the wort to become more fermentable. Beer yeast is capable of fermenting a simple sugar solution (with nutrients and aeration) down to near 1.000 or lower, and does so in mead fermentations where the OG is not too high. (I'm not proposing beer yeast for most meads).
adding enzymes (beano) or other diastatic enzymes (see your LHBS, they may carry some) post boil will shift the sugar balance toward being more-fermentable. I normal brewing we have an easy and effective waay of stopping this action, the boil.
The issue in adding post boil is how to stop it. Some brewers say you don't need to. I do not add enzymes as part of my brewing process so I can't help there.
The S04 yeast cake will be fine to use. Most of the yeast character (or lack thereof) has allready been impartted to the beer, so you will see minimal flavor contribution from the yeast.
You have what should be a very good full bodied English Barleywine going on. One of my best has a 1.065 FG in a 17.5% English BW (National winner of the COC Imperial Anything competition) so don't be afraid of a high FG.
Read this http://beerdujour.com/Howtobrewabigbeer.htm