The best (worst) part is: they don't even need to use actual company funds. Thanks to our wonderful (awful) central banks, they can have the company borrow huge amounts of money at almost zero interest.

Outcomes: the company is more leveraged, executives make huge personal profits without doing anything meaningful, central banks observe the increased risk to the system (higher leverage) and decide they must keep rates lower for longer... inducing the cycle to repeat ad nauseam. It's an integral part of the monetary doom loop we're living through.

If we ignore shady deals, that I suppose happen, Central Banks rational for keeping the interest rate low is to facilitate investment in the economy when there is not risk of inflation (the real economy is under-performing). Keeping the interest rate low facilitates lending, that's the point. Hopefully, some of that money will be use for productive investments.

If a company can borrow money it's because the lender (a private bank normally) considers that operation makes business sense (for the lender). It's the company and the lender the responsible parts for that deal, not the central bank.

Yes, that's one of their stated reasons (there are others). But it hasn't worked for a long very time - the net effect is well and truly negative by now.

My view:

Deliberately stoking massive asset price inflation and wealth inequality while preventing wage inflation is a major cause (along with offshoring) of the social and political tension across the developed world.

Our glorious central bankers (with a few honorable exceptions) still believe that this pursuit of the "wealth effect" is the right thing to do, and the benefits of their monetary diarrhoea will eventually trickle down to everyone. It's not happening and it won't happen.

Some governments are waking up and looking at unifying fiscal and monetary policy. Some are even directing where the credit goes. Frankly I think "independent" central banking exists today in name only, and may disappear within 10 years.

So prepare for inflation to return, to "surprise" everyone, and for our central banks to ignore it and keep suppressing rates right up to the bitter end. And when the rates do rise again, all the zombie firms, malinvestment, bezzles, and unpayable debts will float to the surface and we'll have a bigger version of 2008 to deal with. And a pensions crisis to follow.

Well, exchanging cash for an overvalued stock does not drive up prices for long. I have mentioned negative examples above

It only needs to be long enough to sell...

I wonder if there is any measurable impact from share buybacks propping up or raising price which increases market cap which causes index funds to increase their holdings which then also props up or raises the price, etc.

No it doesn't. There are fewer shares in the company, but the company is worse less. There's no good reason to think that in general the remaining shares in a company will be worth more or less after buyback than what they would be worth if the company had simply held onto the money.

Typically, when there is demand the price goes up, the stock price is not hard linked to the company value, it can lag, lead or even temporarily be correct but generally speaking it takes a while for an over or underpriced stock to catch up to the real value, long enough that there are whole industries dedicated to spotting these differences.