1) I use Quicken and love it. Overall, it is fantastic (IMHO) though there are occasionally times when it gives me fits.
2) Short answer - maybe. Long answer - If Quicken can identify the transactions as being the same, it will not double enter them. But sometimes it will create a second entry because it thinks they are different - usually because the amounts are different. I run into that helping my parents with their Quicken. The bill might have been $48.53 and will get entered as $45.88. In cases like that, Quicken will enter the downloaded transaction as a new and different transaction. There are other instances where it will also attempt to create a second transaction - for some reason, it has great difficulty recognizing the interest payments I get from the bank in my checking and savings accounts, and it really doesn't like doing a transfer between the accounts. But for most all business transactions, the only duplications I find are where the amounts are mis-entered.
3) Yes. It will just register as an uncleared transaction. I actually have about 6 dummy accounts that I use that with. Insurance, Christmas, Vacation, Memberships, Taxes & Registration, and Repairs. I "transfer" money into those accounts each month - enough to cover the estimated bill when it comes due. Then each month, I update the "transfer" to the current total, and delete the old transfers, so I don't wind up with bundles and bundles of unreconciled transactions. For example, I transfer $50 each month for Christmas. So this month, I will show that I transferred the $50. But in February, I will change the transfer to $100 and delete the one from January.
Hope this helps, and if you have other questions feel free to ask.