Frequency of Encountering a Double-Down Opportunity, Expected DD Win Rate

I have done some searching on 'the Net,' and have found conflicting #s. Let's say I am playing a single deck game whereby the dealer must hit soft 17 and I am allowed to double on any two cards (and assume 'traditional' splitting rules for this type of game).

1) How often (to the nearest hundredth - do you like that spelling, Don?:)) can I expect to encounter a double-down opportunity?

2) How often can I expect to win such an opportunity? 55%? 58%?

3) Finally, is the incidence of getting a blackjack the same for the above game as it is for a 6-deck shoe - 4.81% (otherwise expressed as 1/21)is it?