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Lab experiments are not natural. You say yourself they control the environment artificially. By definition, they are artificial selection. There is also a lack of recourse to any feedback & interaction with other lifeforms which could be present in a natural environment.

But speciation has been observed in labs. Surely this is proof that natural selection can occur. The results might be different in the lab than in nature, depending on what the environmental factors are, but the process still happens.

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In nature, natural selection is only observed in the past tense. It's used to create a story of how such-and-such may have taken place. But science requires predictive power. Anyone can use natural selection to explain any past result.

We've watched it happen in modern times. Why would we now have antibiotic resistant bacteria and pesticide resistant weeds if natural selection were not occurring?

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To scientifically observe natural selection in nature, one must evaluate fitness and make predictions. Who can confidently make these evaluations?

As regards pathological bacteria in the gut: Which ones are going to be "fitter" in that environment, those that get killed by antibiotics or those that are resistant? It's not always so hard to see which traits have turned out to be successful adaptations.

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Without natural environmental stress, there is no struggle. The "unfit" thrive right along with the "fit".

Show me a single species that is unaffected by disease. Most also are prone to predation. Natural selection says that the individuals that are best adapted to the environment will have the most offspring, and over time their genes will be the ones which survive in the population.

There's also the idea I've been discussing that organisms are always adapting themselves to their environment. And there's the morphic fields idea.

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"Sexual selection" can work at cross purposes with natural selection.

Obviously they need to be in balance. If an animal's mate-attracting characteristics cause it to be less adapted to its environment -- say, more prone to predators -- then that needs to be weighed up against mating success and number of offspring.

You might be interested in this article: New Evidence That Natural Selection Is A General Driving Force Behind The Origin Of Species

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If you agree that observations of self-regulation run counter to Darwinian natural selection

I'm not sure that you've made a case for a completely self-regulating population, as can be seen in your conversation with LinearAQ. I also think that organisms adapt to their environment even when there are no marked stresses on the population. Organisms and their environments are very tightly bound together.

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we could examine the Neo-Lamarkian paradigm to see if it can operate without struggle.

We already have. And if there's merit in it, which there might be, then it would work as a part of natural selection. It just doesn't involve genetic inheritance.