Actually you didn't really overdose with DAP, You added somewhere around 20 grams in a 5 gallon batch which is about 200 ppm nitrogen - that's not an overdose, but in the future you may want to use some yeast energizer (such as Fermaid K) that also provides amino nitrogen as well as vitamins as minerals that are essential to prevent hydrogen sulfide production such as pyridoxine and pantothenate.
On the other hand, the metabisulphite was a large overdose. While it was clearly not enough to prevent the yeast from functioning, it may be enough to cause the yeast to stall early as the alcohol increases. The pH may also be a factor, and it you check it and find it low, it can be adjusted upward.
If the smell you were getting was that of a burnt match, you may just be be smelling SO2, and if it is blowing off, that is probably a good thing given the overdose. After racking, that smell will likely fade more.
If your yeast doesn't go any farther, you have a couple of choices - either enjoy it as a sweet mead that has around 10% ABV (which may be just fine), or try to pitch another yeast to restart this batch.
"Our results are merely the result of carefully managing the transformation of bee spit into yeast excrement."