I think it is common for first meads to be under attenuated because the must is low in nutrients and the CO2 needs to be driven out of the must to allow the yeast to finish the job.
I make "under attenuated" meads on purpose, regularly. I like low alcohol sweet meads. Back-sweetening does not give the same flavor profile. Never tried driving out the carbon dioxide. Seems like taking a chance on making a sherry-like mead by introducing oxygen at the wrong time.
Staggered nutrient additions and de-gassing have done wonders for my meads. Full attenuation, yet they retain a certain "sweetness," even though the gravity is low. De-gassing won't introduce any off flavors as long as it's done during the first few days of primary fermentation. I have a wand that I hook up to a power drill and let 'er fly. Trick is you need a large 7.9 gallon bucket during this time. The foam will be incredible and you might lose some must otherwise. Once fermentation slows, I transfer to a carboy. There's an excellent article by Steve Piatz called "Making Mead the Easy Way." I'm sure you could find it on Google. I highly recommend the article.
Should I use a 7.9 gallon bucket in the beginning to be safe.( for 5 gallon batch) I was going to use one of my 6.5 gallon glass carboys and am worried now about the foaming and ability to aerate.