1: Use the iPad charger to charge both units, not vice versa
2: The units won't overcharge. Charging stops when it reaches full charge
3: It is quite fine to use while charging
4: Not sure if it is the only unit available, but if you are getting a second charger, get the higher wattage (iPad unit)
1, You can use the iPhone charger with the iPad, but as it's only half the power (5W as opposed to the iPad's 10W charger) it will take a lot longer to charge - especially if you are using it. The iPad's own charger is the most efficient way to charge the iPad. You can safely use the iPad's charger with the iPhone and/or iPods (http://support.apple.com/kb/HT4327)
2, no, when it reaches 100% it will stop charging (not sure abiut third-party chargers)
3, yes - I do it a lot of the time and haven't had any issues or problems.
4, there are third-party chargers available, but I would tend to use the Apple ones
In terms of taking longer to charge, my original iPad took about 4 hours to charge to 100% from empty, and I assume that the iPad 2 and the new one are the same (haven't emptied my new iPad yet so I haven't been able to check). The battery is larger capacity than the iPhone and therefore takes longer to charge to full.
1) Is it okay to use the iPhone4 charger to charge the iPad, and vice versa?
I would DEFINITELY not do that.
2) Is it possible to overcharge the iPad or iPhone- where if it sits on the charger for too long, it's bad for the battery? I sometimes plug in to charge before bed, but I'm sure charging is complete before I wake up.
3) Is it okay to use either/or iPhone4 or iPad while plugged into charger?
4) I would like to buy an additional charger to leave at work. I saw one at Applestore that is $29.99, or so, that is 10 feet long. Is that the only one available?
As far as I know, yes.