I would say #1 is less likely. There isn't a ton of timber harvest of USFS land anymore, much -> most seems to come from private timber lands and BLM lands as well as state forest lands now, none of which USFS has any responsibility or authority for.
I'll mix #2 and #3 together because I think #3 presents the situation with fair accuracy, moreso than #2.
#4 is definitely true. Moreover, many -> most of their "biologists" are on the forestry side, not the wildlife side. Typically wildlife issues on federal lands of all sorts go to the US F&W service, not the land management agency. My expectation is if USFS were to get involved, they would quickly pass the torch to the federal wildlife agencies, not a timber / recreation agency. Oooh oooh .. that brings up another thing: USFS now spends more time / money managing recreation than they do managing timber harvest. ... and if you look at the condition of our USFS-maintained trails you'll see even that is a failure. Since you had your sighting up behind Prospect and Butte Falls, you're somewhat familiar with the area. Did you know that 2 years ago USFS only had 2 people on the High Cascades (Prospect) ranger district tasked with trail maintenance because their budget was so tight?
#5 -- I don't think BF exactly puts jobs in danger, BUT I do think making a fuss about bigfoot might cause problems for someone trying for a career that goes beyond a local ranger district or forest. To move up in rank above .. I think it is GS-11 but it might be GS-9 .. you almost have to transfer to another forest. The supervisor on the "receiving" forest can accept your transfer or reject it. If you've made trouble in your current position, forced the district ranger there to deal with issues they don't want to, then likely the transfer .. as well as the promotion going with it .. are going to be denied. That doesn't mean you get fired, you just get .. stuck in place with a lot fewer upward mobility options.
#6 -- I don't think the president or cabinet have a role in this. This has gone on since 1967 or before across administrations, across parties. If it were as simple as that, with this much time passing, some administration would have decided to spill the beans.
#7 and #8 are both possible.
So to summarize, I think USFS doesn't get to the bottom of this because it is not their job. I think it would more clearly fall on USF&W servivce on public land in general, possibly on some of the military base command (like JBLM) where appropriate. More likely than some deep conspiracy to keep us in the dark, I think the reason lies somewhere else. We, as citizens, expect our government to be subject matter experts, to manage situations, to control them, and to protect us. Facing a situation they do not have expertise, cannot manage or control, and cannot guarantee our safety, the most rational option for their employees is to pretend the situation does not exist. Until someone officially and publicly acknowledges it, someone with the authority to speak for an agency when they do so, the situation truly does not exist so far as their responsibility to handle it goes.
Just my view of course.