Since Mavericks update I get "Welcome to Mail" every time

Question

This is just one of many issues I'm having since updating to OS Mavericks. As a long-time Mac user, it is quite annoying. It's the first time I've felt like a frustrated WIndows user who just installed and inferior OS. Can I go back? Are there real fixes for these issues?
So, Every time I open the Mail app, I get a "Welcome to Mail" dialogue, as if it is the first time I'm using Mail. Here are a few other Mail issues:
No option to reply solely to message author when responding to group/list message. Now I have to compose new message and paste message in, or reply to group and then change “to” field.
Can’t “mail PDF” without opening mail first (or I get error message)
MAil often doesn’t fully quit although it appears closed in dock (I have to force quit)
New mail indicator doesn’t always appear on gmail account
Spell check on mail is now more aggressive and changes words I don’t want changed…setting for this?
I can’t reverse pinch to increase font size on documents/mail
I'm also having issues with Safari, iPhoto, and iTunes...as well as occasional glitches elsewhere.

Answer 1

MAil often doesn’t fully quit although it appears closed in dock (I have to force quit)
Please follow these directions to delete the Mail "sandbox" folders. In OS X 10.9 there are two sandboxes, while in 10.8 there is only one. If you're running a version older than 10.8, this comment isn't applicable.
Back up all data.
Triple-click anywhere in the line below on this page to select it:
~/Library/Containers/com.apple.mail
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a folder named "com.apple.mail" selected. If it does, move the selected folder—not just its contents—to the Desktop. Leave the Finder window open for now.
Log out and log back in. Launch Mail and test. If the problem is resolved, you may have to recreate some of your Mail settings. You can then delete the folder you moved and close the Finder window. If you still have the problem, quit Mail again and put the folder back where it was, overwriting the one that may have been created in its place. Repeat with this line:
~/Library/Containers/com.apple.MailServiceAgent
Caution: If you change any of the contents of the sandbox, but leave the folder itself in place, Mail may crash or not launch at all. Deleting the whole sandbox will cause it to be rebuilt automatically.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.