I don't know too much what is in internet. But I think it's not possible to find good scientific works there. You found some good links. These are about monuments of 16 cent mostly. This online dictionary is very good. It is for real today's Prussian. There are some mistakes as for spelling but nothing terrible. If you need some help about Prussian language, you are welcome but as for internet, sorry I don't know too much. I prefer books.

The Baltic languages are divided into Western (or Peripheral) Baltic (Prussian, Yatvingian, Old Curonian) and Eastern (or Central) Baltic (Lithuanian, Latvian).

As for dialects it is not so easy to write about here. Dialectology is rather complicated subject. You need to study it properly to understand better.

"how the national languages were formed from tribal dialects?"
- there were group of linguists in the beginning of 20 cent who did it, decided what is authentic, more archaic and what are borrowings, wrong constructions and so on.

"And why is Iranian influence?"
- nobody knows why. Scientists guess that it was some invasion about 6th BC to some southern part of Western Baltic tribes. And proto-Slavic appeared as a result of "iranizacija" and "italizacija" of Western Baltic dialects. See works of akad. Toporow and Martynow. If you take away Iranian features from proto-Slavic, you'll see that it will be actually "Prussian" language.

Basel Epigram is the most old monument of Baltic languages written by Petrus Frum heard from some Prussian student in 1369 in Prague. It is a phrase written as a joke in this old book found in library of Basel. That's why it is called like this.

To compare all Baltic languages is not so easy now. It would take plenty of time. In Russian you can find books of Dini and Toporov where are the describtion of Baltic languages.

About phonetics, interesting thing that Baltic languages still have tonic accent. From Slavic languages the only Serbian language still has it. Prussian has the similarity with Latvian in this way while in Lithuanian all tones are in the reverse way. This shows that Prussian and Latvian have more archaic Western Baltic phonetic features. But Latvian has fixed stress. This feature is not archaic.