You have a bunch of questions here so I'll cover them individually.
So if its a less likely reason, I don't want to take off a bunch of line and sacrifice casting distance.
You won't be sacrificing casting distance (you can't cast the whole spool) but you could run into problems if you hook a big fish.
You should have a 1/8 inch gap between the line and the edge of the spool.
... is there a way for me to tell now if it is spooled incorrectly? (I'd hate to have to remove 200m of line by hand especially if again that is not the cause.)
The only way for the line to come off the spool is the way that it was put on by the reel.
I suppose it could be put on "backwards" but the first time you cast out and reel the line back in, it will go back on in the proper direction.
The bail moves only in one direction when reeling in and they all probably turn the same way so the store probably put it on properly.
I am also wondering how long it would take the line to lose its original memory and get 'retrained'?
It was suggested to put the top part of the line in water to help retrain it. But aren't polymers hydrophobic, ie: shouldn't they repel water and not be affected by it (after all the line is designed to be spending much of its time in water)?
He was probably suggesting putting the line in warm water to soften it up then cooling it "flat".
Please explain how my bail could be "in the wrong open position prior to casting" and could something else in my casting technique be at fault?
It is possible that the bail isn't open completely and is unlocking the spool allowing it to unravel the line.
Or, there is some other mechanical problem with the reel that allows it to unspool.
And back to my technique, if I am planning to be casting spinners,
lwet flies and bait under float using a heavy 10' fiberglass rod what style/technique would give me the greatest casting distance with reasonable accuracy? Given that the rod is rated at 2 3/4 oz (80g) and that I have a 20lb test line, what casting distance can I reasonably expect with the above rigs?
Things that affect casting distance are:
rod flexibility - 10 foot fibreglass rod is probably not very flexible - the rod needs to "load" to cast well
line wieght - heavier line does not cast as far
spoon wieght - a spoon/spinner that is too heavy or too light won't cast properly
Other random comments:
20 lb line might be too heavy - most pin guys use 20lb but that has to do with casting a pin more than anything else
I would probably use a 15lb "limp" line as suggested
You could take it back to the store and let them have a look
(Rod: long replies are a nightmare in the tiny edit window)