if you've experienced a 50% decrease in your fuel mileage - something is seriously wrong with your car.

...Perhaps your initial mpg numbers were somewhat inflated? 45mpg is hybrid territory, not 14-year-old delsol with a d16 territory.

As far as your transmission problem is concerned, it sounds to me like you need to bleed your clutch master/slave cylinders some more. The clutch isn't being depressed entirely, which is why you find the partial grind while shifting, and the difficulty in engaging the reverse gear.

good luck
-jonathan