Not sure there is a definitive, one-size-fits-all answer, but I can give you my experience.

I've had much better success with multiple touch points than the single blast. I would specifically target folks who opened, but didn't click the first email, and send them a second email with a modified message.

If you have any more details, I can probably dive a bit deeper.

Sending a smaller list multiple emails in a series (NOT the same piece multiple times) will work best. I would definitely not keep sending exactly the same email as almost anyone who actually opened more than one would consider that spamming.

It depends on your market, product, and a lot of different variables. Direct mail is one of the forms of marketing that you NEED to have an expert handle for you.... unless you want to lose massive amounts of money.