Since there's no way to tell the stats of my units, if I upgrade, say, Phalanx defense, does that apply for the old phalanxes I have or only the new ones?

Also, the defense upgrade only applies to Phalanxes built at that village, right? So if I have phalanxes from other villages stationed there or stationed at their original village, they only get the upgrade from the village from which they were built, right? Otherwise, I've been wasting a lot of resources upgrading Phalanx to level 11 defense on two of my villages.

The new armor goes to all phalanx trained at that village. If your phalanx are reinforcing someone or one of your other villages, they will get the new armor when they return. Don't worry if you have built up armor in a village that doesn't have phalanx because when you get attacked, you will make as many troops as you can as fast as you can (phalanx) and the armor will be ready for them. Very good to have level 20 armor in your main village for your phalanx .... and druids too if you can.

Hope this helps...

You can find more information about armor and weapons in the travian faq links and the travian forum