> But certainly if the GM entity had $10B more cash tomorrow, it might not be worth $10B more, but certainly it would be worth $5-$10B more (or at the very least >0 more)?
No, again, that's not how it works. Shareholders value companies primarily based on a multiple of earnings with an overwhelming tilt toward future expectations, not based on cash. Cash can receive anywhere from a medium to a near-total discount, depending on the context & company. As an easy example: Apple's cash has stopped growing at the rate it used to (they're paying a lot of it out), while it has taken on immense debt in a short time, and its earnings have not expanded in years (in fact they've fallen as with sales, re fiscal 2016 vs fiscal 2015) - meanwhile, its stock has been given an increased multiple lately, AAPL is up 47% (adding $200+ billion in market cap, while earnings went down and debt went up) in the last year despite those theoretical negatives. Why? Shareholders mostly don't care about the accumulated debt or the slowdown in cash accumulation.
There are a few exceptions, for example if a company is under serious bankruptcy risk. That can dramatically damage the valuation given to a company via its income. In that specific case, adding a ton of cash to relieve the bankruptcy risk, can produce a greater than 1x release in value as shareholders shift back to operating fundamentals & future expectations. Valeant Pharmaceuticals is a walking example of that right now. To the extent Valeant manages to lift the dark cloud of bankruptcy off of it, their market valuation will rise (every time they announce they've paid down some debt, or sell an asset for a good price, their stock tends to spike; they have something like $29b in long-term debt and the debt interest is threatening their existence; if they magically added $10 billion in cash tomorrow, their market cap would expand by something near a 1 to 1 basis or likely greater (depending on the bankruptcy pressure, relief from it can generate an amplified upside)). Another exception example: Las Vegas Sands (LVS), the casino giant, was under serious risk of bankruptcy during the economic crash in 2009; its founder, Sheldon Adelson, decided to back the company with a large amount of money, the stock proceeded to increase by 5x to 6x in a matter of a few months afterward, due to the relief that bankruptcy was no longer nearly so likely (shareholders shifted back to valuing the company on its future earnings/growth/operating expectations). LVS gained so much so quickly, because the risk of bankruptcy was viewed as being so severe it had drastically depressed the market cap of the company.