You''ve pretty much answered your own question. I probably have the same unit (mine is the GPS Map 76S)
Anyway, the way I understand it is this, the reason they set the default speed for using the gps for compass readings at 10 miles an hour is that this is about what it takes to get an accurate reading between gps sample times (maybe you can reduce the frequency of how often the unit tries to update it''s position information) and you will have moved sufficiently between readings that it can deduce your course.
However, you shouldn''t have any problem using the compass at lower speeds, but in the directions, it says pretty plainly that you must hold the unit horizontal or the reading will not be accurate. I image the same thing would apply to a hand-held magnetic compass.
Don''t really know if the internal compass can be thrown off by electronics or metal, but I doubt it, as the unit itself is electronic, and that would be pretty hard to engineer.
Make sure that you''ve calibrated your compass. Go into one of the setup menus and select calibrate, then it will ask you to rotate your unit through two horizontal revolutions. I think that there is also a place where you can enter deviation, though it''s not clear to me why that should be necessary (they should know where you are, and have that info available to the unit)
Oh well, good luck.