1. No. It stops at the BWI Rail station, which is not accessible
to the airport by foot. From which ...

2. Up to 15 min wait for a bus, which then takes 5-10 min. If
you are really in time trouble, you can take a cab from the
rail station to the terminal. This saves you the wait for the bus
and a couple minutes more if you are not flying Southwest.

3. Doing a quick and dirty calculation, allow 65 minutes from
Union Station to the ticket counter, just at a guess.

4. The best I can come up with is http://www.amtrakdelays.com

A guide to prices for afternoon trains:
Amtrak regional $18 to 21, about 25-30 min to BWI rail station;
MARC commuter train $6, about 35-40 min.

You may buy MARC tickets from the Amtrak machines and windows.