»»...singing with the back of your mouth still open...you will have to close it up to make consonants...««

You don't have to. I've had a much smoother sound since I learned not to. It felt VERY peculiar at first--keeping the back wide open while lips, teeth and tongue make consonants--but like every new use of muscles it got more comfortable the more I did it. It took me about 3 years of not-very-dedicated effort to make it second nature. Six months of that was learning to hold that space open without breaking into a yawn. It might have taken me less time if I'd focused on it daily. This open back-of-mouth (top-of-throat) also eases quick, deep breaths, which is very helpful in songs that don't contain long breathing spaces.