Yes, I realize that Marx's labor theory of value is not popular nowadays. I think that is a mistake. I think even investors would get a better descriptive model of reality if they adopted it for their own uses. That is what I am trying to do myself. I could care less about overthrowing capitalism. Instead, let me milk it for all I can....

As for "labour crystallised in the product," that's not how I think of it, regardless of however Marx wrote about it. (I'm not particularly interested in arguing from quotation, nor would you probably find that persuasive, so I'll just tell you how I make sense of it).

I interpret the labor-value of something (good or service) as the relative proportion of society's aggregate labor that must be devoted to its production in order to, with a given level of productivity of labor, reproduce that good or service sustainably over the long-term. Nothing gets crystallized in any individual product. That would be downright metaphysical thinking.

After all, just because an individual item has a certain labor-value doesn't mean that it will individually automatically fetch a certain price. It is not the individual labor-value that influences price. A pair of sneakers made by a factory that is half as efficient as the typical sneaker factory does not have twice the labor-value or fetch twice the price. What matters is the "socially-necessary" labor expended on an item. And how can that be perceived? On average in the long-run, if a particular firm's service or production process does not yield an average rate of profit, then that is society's signal, after-the-fact, that some of the labor devoted to that line of production is not being counted by society as having been "socially-necessary" labor. (Of course, technological change can lower the socially-necessary labor for a certain line of production, which will appear as falling prices (assuming a non-depreciating currency) through competition and below-average profits for any firms still using old techniques that waste labor that is now socially-unnecessary).

If business owners were to rely on a crude, metaphysical interpretation of Marx's labor theory of value that assured them that the value was already baked into their product as soon as it rolled off the production line, they would be unpleasantly surprised if it were to turn out that they could not realize the expected labor-value in their product...perhaps due to something like their competitors having, in the intervening time, embarked upon a technological innovation that changed society's unconscious, distributed calculation of what labor was "socially-necessary" for this line of production....

As for your final questions: it's a bit complicated, to say the least. There are even various schools of Marxists that don't agree with each other.

I think there is somewhat of a consensus that there is a real long-term tendency for the (real, inflation-adjusted) world rate of profit to fall, theoretically and empirically, and therefore you can expect there to be an ever-decreasing ceiling on how high (real) interest rates can go during a business cycle before they begin to eat up all of the profit rate and leave nothing for net profit of enterprise, thus precipitating a decline in production and a recession. (Although some Marxists reject that there is a theoretical or empirical tendency for the rate of profit to fall. See Andrew Kliman's book "The Failure of Capitalist Production" if you are interested in this "exciting" debate).

More controversial still is the question of what, if anything, monetary policy can do to influence interest rates and aggregate purchasing power to prevent future recessions. I concur with what I call the "Commodity-Money" school (see Ernest Mandel's work on "Marx's Theory of Money", Sam William's "Critique of Crisis Theory" blog, or the writings of Jon Britton) that argues that there is actually very little that monetary authorities can do to alter the course of business cycles because paper currencies, while they are no longer legally tied to commodity-money, remain tied to commodity-money in a practical sense, and that movements in the world production of commodity-money place practical limits on what authorities governing paper currencies can do.

I don't have the patience to explain all of this here in greater depth when others have already done so elsewhere. Sam Williams's "Critique of Crisis Theory" blog is what I would recommend reading from the top to get the clearest explanation of this stuff.

By the way, my "commodity-money" understanding of Marx's labor theory of value leads me to believe that we are currently entering a boom phase in the business cycle in which equities, on average, will continue to perform well. (I have holdings in Vanguard Total World Stock (VT), for your information. It is a very simple instrument for tracking the world economy with low management fees). So, expect accelerating growth for 3-4 years. Towards the end of that period, I expect an oncoming credit crisis and recession to be heralded by world gold production to start declining slightly and interest rates to be inching upward to a dangerous level infringing on the net profit of enterprise (hence, why a theory of the expected average rate of profit is so useful!)...with little that the Federal Reserve or other monetary authorities will be able or willing to do about it due to the fear of depreciating paper currencies with respect to commodity-money too much. Business will continue to apparently boom for a short while longer, but it will be in its unsustainable credit-boom phase by that point, and it will be time to cash out of equities and into commodity-money (gold).