A few points to push back on...
The majority of the record low participation rate is explained by aging demographics as the baby boomers reach retirement age, which had been expected ahead of time by macroeconomic models. Some of it is excess slack in the labor market unexplained by the U3 sub-5% rate, as has been the PTER rate (difference between U6 and U3 rates), which was a reason the Fed cited for holding off on raising the target rate after having achieved U3 unemployment historically consistent with full employment / the NAIRU. In other words, there was more to the story than just U3 unemployment, which is sort of consistent with what you're saying, except that a lot of that excess slack has now been absorbed and we continue to print monthly payrolls well above the amount we need to keep unemployment stable. Also, contrary to what you're saying, the Fed was very vocal and explicit about this excess labor market slack when it was a real issue.
Low levels of corporate investment are certainly a problem, and they are contributing to low forecasts of long term potential growth. This issue does raise a problem with monetary policy. Central banks can control interest rates and liquidity all along the yield curve, which essentially gives financial asset holders cash but doesn't force them to do anything with it other than continuing to hold assets or buy more financial assets, which given the performance of financial assets under an easy monetary regime can be a hard incentive to break. You probably agree with me up until now. Where we break in agreement is that I think there is a solution, rather than just being in a hopeless situation. The government should become an investor of last resort, borrow money at a near zero rate of interest, and invest in capital projects to improve the country's infrastructure. The bar for the government to earn a positive return to the real economy is quite low. Across the world, monetary policy is very loose and fiscal policy very tight. Most central bankers are critical of the current global attitude toward fiscal policy, and I agree with them. Central banks currently feel they have to ease more largely to make up for harmfully tight fiscal policy.
I don't think candidates like Trump are a coincidence, either. As per the paragraph above, prolonged easy monetary policy widens income inequality in a trickle-down economics sort of way. If the asset holders who are given the cash don't engage in real investment with it in favor of financial investment, the working class never really sees the benefit as financial asset gains far outstrip wage inflation. I think that's why you see an analogous political climate in many European countries. Europe has had a wider divide between monetary and fiscal policies than the US. But again, no one has ever argued that monetary policy should affect macroeconomic variables other than aggregate employment and aggregate inflation. It is up to the government to sort out the breakdowns of those variables to a healthy place, and governments just aren't doing that right now.
It sounds like your argument is that the Fed is squeezing the last bit of growth out of the economy before we're all out of it. I just don't think that is correct. The Fed has a dual mandate, unemployment and inflation. Maybe there's an unofficial third mandate of managing stock and bond market volatility. That's it. The Fed doesn't have a mandate to target investment or growth in general. That bit is up to the government. The BOE in 2013 did their FLS and the ECB today is moving toward credit easing. Maybe that's what the Fed should have done, namely monetary policies that target the real economy rather than broad macroeconomic variables, but again that's not been the Fed's mandate. And further, in those economies, the central banks beg fiscal authorities to loosen their stances. I look at the investment problem as more of an opportunity than you do. If corporate investment were strong, and we still couldn't generate productivity growth or expectations of long term potential growth, then that would be a much bigger problem. Instead, I see a system that accidentally disincentivizes real investment in favor of investing in financial assets. If that set of incentives were changed somehow - I'm suggesting a mix of fiscal spending and government policy, but maybe there are other options - then real investment would flow through the economy and we'd experience genuine real growth.