I don't think im over-estimating the ease of fighting a guerrilla war. But i do think you are over-estimating the ease of fighting a counter-guerrilla war and is one of the most difficult forms of warfare to carry out. And yes you cite examples of successful counter-insurgency. However bear in mind that there are far more examples of successful guerrilla warfare then counter-guerrilla warfare.
Before you have a counter-insurgency you have to first have an insurgency, and with the right occupation strategy this can be avoided, regardless of whether the country you're talking about is one where people own hunting rifles, revolvers, and shot-guns in significant quantities.
And I never said summed up any argument as these countries have more guns then these do. However, it should be noted that when you have a nation that is determined to resist any invader and has a heavily armed populace (Switzerland and USA for example)
It's easy to airily state that countries that have never actually been invaded in their modern history (though both have seen civil wars in which insurgency was not a major factor) are "determined to resist any invader". People in every country believe this to be true of their own country except where glaring historical examples disprove this (though they are often given to simply inventing a myth of fierce resistance even if these exist).
it is much more difficult to invade and occupy said nation when the population is already armed and determined to resist. Where as a population that is largely disarmed (USSR until partisan support began, Belgium and Czechoslovakia) it is much easier to occupy.
I think you need to read up on all three examples here: guns of the types under discussion (hunting rifles, shot-guns, pistols) were fairly easily available in Czechoslovakia and Belgium. In fact they're still relatively lax as far as guns go in the Czech Republic at least - I've been shooting on ranges a few times there (both times in what was essentially a gun-pub) and everything except fully automatic weapons seemed to be on offer. Czech citizens can buy and keep guns fairly easy even now which is why new EU proposals to tighten up gun sales across the EU are especially controversial there (and in Finland, for the same reason).What's more, both Belgium and Czechoslovakia had had conscription in place for a long time, with large numbers of men being trained in the use of firearms, and significant numbers of combat veterans living in both countries.
The reasons why both Czechoslovakia and Belgium saw relatively little in the way of partisan resistance (barring the last-minute uprising in Prague at the end of the war) has little or nothing to do with the availability of firearms in those countries, and much more to do with the skilful use by the Nazi of credible collaborators (e.g., the Rexists in Belgium, the Hacha government in the Czech lands) and the relatively lenient nature of the occupation of each country coupled with the threat of massive retaliation for acts of resistance. I'm not going to go into discussing banned topics, but
the aftermath of the assassination of Reinhard Heydrich is well known. Of course, the absence of remote areas in these countries where partisans could establish bases has to be considered as a very important factor in this as well.
For the Soviet Union I'm not sure you're familiar with what actually happened there. Partisan resistance in the Soviet Union began fairly early - guys like
Sydir Kovpak (who gained his spurs as a partisan in the Russian Civil War) decamped to the forest areas at the time of the invasion. Belarus had more than 200 partisan units operating by August 1941 - these were pre-planned by the Soviet authorities. By late 1941 there were already 7,200 partisans operating in Belarus alone.
There was no delay in partisan warfare breaking out in the USSR-proper, nor was the populace "disarmed" in the face of the invasion - indeed they were armed in the face of the invasion. What did occur is that those areas which had recently been annexed by the USSR, or which had suffered particularly harshly under Soviet rule, initially welcomed the invaders until the invader's harsh occupation policies turned them against the axis. What also happened is that Soviet partisans were relatively inactive in 1941, not because they lacked arms, but because they lacked the organisation, logistics, and leadership that was later given to them by the Soviet central command to make them part of their overall strategy.
That includes collaborators in both examples which depends on whether or not the occupied nation accepts them or rejects them.
I wouldn't instantly assume that the United States would lack collaborators who might be seen as a credible government by a significant percentage of the population in the wake of a successful Axis invasion. The defeatist inclinations of people like Joe Kennedy, Lindbergh, Robert R. McCormick is well known. America First had 800,000 members. Without going into the societal problems of the time, the US had societal divisions and opinions not too dissimilar to those seen in Belgium that an intelligent invader could have exploited.
Now we all know many Belgians wanted to resist and the Czechs never wanted to be absorbed. However if the population was as heavily armed as Switzerland or the USA for example, you can imagine the possible headache for the Nazi's it would be to crush the uprising short of any rather extreme measures. Especially before WW2 in the case of Czechoslovakia should it get international attention.
As already explained above, people in Belgium and the Czech lands did have relatively easy access to hunting rifles, revolvers, and shotguns pre-invasion. In the case of the Czechs, the Germans were not hesitant to swiftly supress resistance and the Czechs knew this.
The big difference in the case of the US would not have been the availability of non-military grade firearms which are not, anyway, nearly as useful in guerrilla war as mortars, explosives, radios - the weapons with which guerrilla warfare is primarily fought. The big difference would have been the availability of remote areas in which partisans could have established bases from which to operate and which an occupier would have found difficult to locate and destroy, however, this would not have, by itself, created an effective guerrilla movement. The ideal situation would be a pre-planned resistance organisation similar to the Soviet one. Barring that, resistance in the US would have had to have coalesced around everything from the Boy Scouts to the Communists the same way it did in other countries.
In game terms this would require the research of resistance technologies and perhaps the construction of "resistance cells" or whatever resistance-modelling units HOI4 will have. The "suppression effect" of occupying forces should be high in high-infra states and low in low-infra states.