My guess is that the oil spot is on the rear shock since the front should have a boot on it and you won't see any oil on the shock itself (unless the air filter is over-oiled in which case it would lead a little, no big deal).
My guess is that the rear rim is dented ("the rim would not accept a new tire"). If you continually see flats on the rear it could be a number of things, but if the bike is put together well (by the shop) that would probably be from running too low a tire pressure.
If your LBS is totally incompetent (which, if cables are constantly coming off the bike and getting tangled in the crank, is a good possibility) it's also possible that they screwed up the front wheel. Maybe they "trued" it once and way over or under tightened the spokes and now they just can't get the darned thing right. Wheels take a competent mechanic to get right.
There's nothing about "Cannondale" that should be causing any of your problems. As others have stated, they just make the frame and in this case the front shock. The rest of the bike is made of the same parts you'd find on other brands.
I suspect you need a new LBS.