About how many images is the Pentax spec'ed for before its shutter fails?

Good question... Pentax never responded when I asked :) None of the camera manufacturers really like to advertise that there is a potential failure mode in the cameras. I'd expect a couple hundred thousand shots...

Can you use larger memory cards or attach to an external hard drive?

The Pentax K110D/K100D can be used with much larger cards... as much as your wallet can buy! We do not have a provision for using an external hard-drive.

What is the most frequent frame capture interval you would trust with the Pentax camera and the digisnap? Could you do an image every 10 s? Or faster?

An image every 3 seconds should be sustainable until the card fills.