nor can we theorize on the probability of the existence of a deity without accurate data,

Yes we can, we theorize that things are unlikely every day, without having proof that they do not occur. A car will probably not drop on your head the second you walk out the door. We have no proof it will not, and there is a slim chance it is possible, but you are not reacting to that possibility.

nor can we hope to understand the limitations at this time of the universe in which we live, or hope

Indeed, so you are suggesting we should react to anything that is possible? Are you right now busy constructing a vast pyramid incase it appeals to the potential alien species known as 'funkmonkeys'? Certainly, we should accept it is possible such a species exists, and that pyramids get their mojo flowing, but it is exceptionally unlikely. Unlikely to the extent that I will certainly not be trying to build a pyramid tomorrow.

An atheist merely lacks positive and negative assertion of belief.

That seems accurate.

All of the scientific body of knowledge that we do have, doesn't necessarily point to the idea of no God,

Nor does scientific knowledge say there are no funkmonkeys, no invisible gnome that lives on your head, and no fairies at the bottom of the garden.

As a matter of fact, if we had definite proof of the existence of God, atheists could still exist from a pure logical point of view.

I am sure we would be using a different term for that group, and it would certainly take a form vastly different from what we currently know as atheists, so I don't really see your point.

People can believe something without having to act a certain way.

They can, but beliefs still can affect decisions. It is not inevitable in every case, but it can, and does occur.

You see, knowledge and belief are largely independent of each other. Something we here in r/atheism try to stress to those not in the know.

I agree with that, I don't really see your point. They are independent, but they can still affect each other.

Now, what if they do truly believe, but also believe that suicide is a mortal sin? Perhaps they believe in an omniscient God, and believe that all of their suffering is a rite of passage to the gates of heaven? And what if by taking the easy way out (pulling the plug, as it were), even if they did not commit suicide, hastening one's death deliberately actually was equivalent to suicide?

Religions are pretty vigilant about trying to prevent people from taking a shortcut to the idea of heaven they sell (for obvious reasons), but their methods are certainly not foolproof. Firstly holy books are so old, and so vague, it opens them up to interpretation in many ways. Secondly, they contradict themselves, so people can simply pick whatever suits them for their current desire. Thirdly, you can simply find a way to die easily that does not involve suicide, such as free soloing, chainsaw juggling, bear wrestling, &c. Actually when it has suited them, religions have actively encouraged dangerous professions (soldier, &c.), claiming that dying in a certain way will surely lead to heaven.

There is plenty of leeway for an early death within religions, the point is, despite that opportunity, and despite the promise of an improved life after death, people do not seek that opportunity. People who really believed would have no troubles interpreting religious teachings in a way that permits them to die sooner.