Yes, I agree, it's what you're used to, not your handedness, that matters. I am also left handed and drive left-hand drive cars, but my right-handed husband is a better driver than I, and I know right-handed people who are worse drivers than I as well.
I don't think so. I am right handed and I drive a left hand drive car. Maybe it is easier for a left handed person to drive a right hand drive car because they can use their dominant hand for the gear lever.
For a period of two years or so, I travelled very frequently by air to the UK and rented a car each time. During that period, I must have worked my way through Avis's complete range of small cars and, because I was unfamiliar with them, the main problem I had was finding the ignition lock to put the key in (this was at Manchester Airport where the rental car pick-up is on Level 11 of the car park and is very dark. Why on earth do they put the ignition on the right hand side of the steering column of right hand drive cars? Sometimes, the only way to find the key hole was to open the driver's door!
I can drive on both sides equally as well- it's just a question of giving yourself a couple of hours to get used to the difference (and actually changing gear instead of winding down the window).