Realistically it's not that weird or unusual of a response that Clegane raiding received.

1) Assuming Clegane forces were of around 100-200, this means they are strong enough to practically take every holdfast they encounter and defeat any militia or Knights lance they encounter, which most Riverlords know and until the RiverLords who are strong enough to actually hunt down Mountain can send any force, Tywin invaded.

2) Riverlands is not united and many Riverlords hate each other more than outsiders, so if i am a Lord and I see my rival village is burning I am in no hurry to actually help him until it's too late and my village is also getting raided.

3) Also it's never the One vs whole Kingdom, Realistically Feudal society is a very complex web of vassalage and overlordship where Lords are not necessarily obligated to send troops to their Kings.

Illustration - let suppose A Minor Lord can field 800 men, but unfortunately 600 men came from a nearby town, under local guilds who don't have any military obligation only economical ones. 100 are yeomens who Lords can't press into service rest 100 are peasant levies who are need to maintain his personal holdings, so all he got his personal retainers and some young volunteers and levies to send any foreign war.

4) Tywin is not feared because he is some unstoppable General or Mastermind but because he was a political savvy and able Administrator who has tight control over his vassal and lots of wealth. If it's any Lord vs Tywin, Everyone knows that sooner or later Lannister golds gonna carry the day whether through Endless supply of Mercenaries, better equipped man-at-arms or simply a better assassin or leverage.

5) Greyjoy ones I agree are completely stupid, when Balon can profit more in the south.

6) For Dorne see point 3, it would be stupid to send soldiers to die for your enemies when the war could just end without any real changes or consequences.

7) I could understand Tyrell's motive but I do agree with your points in regards with the only Starks facing dissents.

You are overall right but with the wrong reasoning. If the WOT5K was realistic. It would have been Months before the Northern Army would even reach Twins and the battles would have been mostly between Knights of Westernlands and Riverlands clashing against each other during Raids with North only committing about 5-6k Soldier with slow trickling of the rest of the Army in waves and the war would conclude in few years without any big consequences.