Lastly, I seem to be having a buoyancy problem

Which part is sinking first?
I can imagine that if you get tired, you start breathing faster and you inhale less air. less air in your lungs is less buoyancy and your upperbody will sink. So slow down just a little bit which makes you less tired.
I have the problem that my legs sink, I have low fat%. So what I do is use my arms for forward motion and use my legs to kick only to get them up and in line with my upperbody.
Well this is my thought, but as I never had any sort of freestyle swim lessons, I can not assure that mine is the good way to swim. So I wonder what others have to say.

My legs are sinking. They are useless dead weight. My flutter kick is worthless. I swam in high school competitively for 3 years and never could get it down. Breaststroke, butterfly and backstroke were my strong suits. Freestyle was always my worst stroke. As a breaststoker, I always regarded flutter kicks as punishment. I think my hips begin to sag in the water after a few laps. My lap times as fast with a pull buoy as without.

you are reaching for air by lifting your head. Simply put as you lift your head, your legs sink. As your legs sink, you reach higher with your head to "get to the air" to combat the sinking feeling. SEE THE CHAIN OF EVENTS HERE?

The key is to drill focusing on keeping your head down in the water to the point that you are almost looking at your toes. Don't go so long that you tire to the point of losing your form! The distance will become easier as your efficiency increases. As you drill, remember to rotate your whole body to the air. Your navel should point to the side of the pool or above the waterline. This will give you plenty of room for a breath.

My .02 based on 9 months of TI Masters Swim,

Paul

The simple law of pool physics - As your head goes up your legs go down. It's inevitable. Every swimmer no matter if theyr'e just starting out or are Olympians have to contend with this simple law of nature. So how do you comabt it? Well you can keep your head underwater for the whole time. Not very practical. You can continue to drag your legs like so much barnacle under a boat. Not very appetizing.The answer might not be in your legs or buoyancy. It could be in your breathing. Breathing correctly can help keep your body in balance. If you raise your head to far up to gulp air your legs will sink. So the key is to breath sideways! Catch the air pocket in the middle of your stroke and then keep on moving. When I swim my head barely comes out of the water. I try to keep the same linear form throughout my entire workout. My breath stroke barely makes a ripple because as soon as I lift my head to far my legs sink. So I reach my arm out and turn my head but my head goes forward not up! I hope this isn't too confusing. And I also hope it helps.