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I am curious whether the current Lambda-CDM model of cosmology matches well with observational data, especially expansion of the universe.

How well does Lambda-CDM defend its established status from other models, such as quintessence (quintessence can be said to extend Lambda-CDM, but there are some models against the standard model, I guess.)?

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It fits remarkably well. One of the defining features of a cosmological constant is its equation of state. The equation of state, $w$, is given by $p \over \rho$, where $p$ is the pressure it contributes, and $\rho$ is the energy density. A cosmological constant has $w=-1$. The WMAP seven year report recorded the value as $w=-1.1 ± 0.14$. Within the error margins, the cosmological constant fits very well.

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We don't know how the relationship between gravity and dark energy changes over time as gravity decreases (from the rest of the universe), because one cancels out the other to a degree we don't know.

It is not reasonable to assume that as the universe expands more strings of dark energy magically appear to keep the density constant.

Einstein originally proposed the idea of a cosmological constant because it was needed to maintain a static, non-expanding non-contracting universe, and he called it the biggest blunder of his career.

There's no way that dark energy can keep a constant density in an expanding universe. If the observations are correct that the universe is expanding faster, it's because as gravity decreases, the universe only needs a smaller push to accelerate.

If dark energy is not at constant density, which doesn't seem likely due to the way other energy behaves in the expansion, and if it changes from a pushing to a pulling force in the future, like when it gets sucked into a black hole, then the fate of the universe changes to an endless cycle.

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