I'll do my best to answer everything:
1: Consonant gradation doesn't apply with most consonant clusters, which means it usually doesn't apply when multiple consonants are next to each other.
posti > postin (not posdin)
pitkä -> pitkä (not pitän/pidän)
Consonant clusters do change though, when the consonant gradation is specifically defined for a cluster, like lt > ll: ilta > illan
And another thing, is that new loanwords don't usually go through consonant gradation, look at the modern word auto (car):
auto > auton (not audon)
2: Here, your word is a compound word of järvi and pää, so the actual question is 'why doesn't pää become väässä (or vään)'
Consonant gradation only applies to words that have multiple syllable. In other words, the first letter never changes. That's why pää just stays pää.
3: Not all cases trigger consonant gradation. The illative for example, always takes the strong stem (so no consonant gradation in this case).
leipä > leipään (not leivään)
ruoka > ruokaan (not ruoaan)
If you have any other questions on consonant gradation or Finnish grammar in general, please don't hesitate to contact me. It was my favorite subject to learn about, and I'll gladly write long replies about that~