The heat death theory assumes life has zero impact on the universe.

Most of the universe is void. Most of the rest is whatever dark matter is. What we consider to be the entirety of material reality is a fraction of a fraction, and out of that, as far as we can tell, Earth has the only life that exists. There's probably more life out there, but if there is, as the Fermi Paradox points out, they're awfully quiet.

I think it's fair to assume that life will have practically zero impact on the universe. Even if the universe was teeming with it, it would be of such little significance at scale that it might as well be a rounding error in reality.

There is merit to your thought process but it could just as easily be that life is the spark that sets off an explosion that irrevecobly changes the universe.

Think of a virus. A handful of specially shaped molecules has permanently changed human society.

What? It does no such thing, it just assumes life doesn't violate the second law of thermodynamics, which seems reasonable considering we've never observed it doing so.

It assumes life is subject to the second law of thermodynamics, which it is.

Avoiding heat death requires a renewable source of negentropy, to bypass the second law. Some sort of perpetual motion machine that you can pump energy out of. E.g. if you could extract work out of the expansion of the universe fast enough to build expansion-work-extractors faster than they decay, and expansion continues indefinitely, then you can avoid heat death.

Life itself is a renewable, exponentially growing form of negative entropy.

We can't predict what life will be doing in 100 years. What will it look like in 100,000,000 years of exponential growth?

You should read the wikipedia article "Entropy and Life" [1]. Life is not a growing form of negentropy. This is why living things have to eat.

1: https://en.wikipedia.org/wiki/Entropy_and_life

How so?