Will deleting pictures from Dropbox online delete them from the PC’s (and iPad’s) folders

This is pretty easy. Simply check the check box next to the folder(s) you want to download and then go to

More Actions > Download Folder

You can also do this to download multiple files within a folder. Check all the items you want to download and then go to

More Actions > Download Items

Without using any kind of API, from where you'll probably able to get the e-mail of your shared folders, you can check the e-mail from the web interface.

Since there is no such thing as contact list here, you can do the following:

1. Login to Dropbox.com
2. Go to Sharing tab
3. Enter any folder and click on Shared folder options button
4. Once you're there click on the Members tab
5. Finally by clicking on the names there you can send them e-mail, and it's using your native client :)

Another way of checking your "list of shared e-mails" is to create a folder, again from the WebUI, then click on the Share a folder and when you start typing the autosuggestion kicks in. I don't know how many mails do you have, but you can test all the letters (they are not too many) and it really depends on how important this is for you, to go through all that hassle.