Hi Cyperium,
I have a few minutes now, and I think I see where your confusion is.
A culture of single cells grown from only one cell would grow large pretty fast, there isn't much difference between that single cell and a new form of life, it is also pretty defenseless.
This could not be farther from the truth. The difference is more extreme than the difference between and elephant a maggot.
Modern cells have membranes or tough cell walls, they can hold in water, and use pumps to control their homeostasis, they have chemical defenses against oxidation, melanin and other pigments to defend against ionizing light, special proteins to prevent damage from cold, the ability to move and sense their surroundings to a small degree, produce toxins and poisons to wage chemical warfare against enemies- make themselves poisonous so they can't be eaten- and even form tough impermeable shells resistant to light, dehydration, freezing temperatures, and heat.
And that's only part of it off the top of my head. Cells are fortresses, evolved over billions of years for survival on this planet- and with arsenals of chemical weaponry.
Early life had none of these things. Zilch. Nada. It was just a self-replicating protein, only slightly more complex than a prion.
Is oxygen not good for abiogenesis? I don't get what you mean by "too much oxygen", why would that be a bad thing for a new form of life?
Oxygen is lethal to all living things and absolutely destroys DNA- it is a ravenous and highly damaging molecule. That is, unless the DNA is protected and encapsulated, and the organism has evolved carefully regulated systems to defend it from oxidation and repair the damage.
Many bacteria are killed almost instantly by the oxygen in the atmosphere- these are the types of bacteria that do not have defenses against it. Look up 'Obligate anaerobic bacteria'
This would certainly be the case for new life.
You said that there are a lot of crossing of DNA so that the lines blur between many different origins. They would probably do that if the differences between the DNA was small, but I don't think that they would blend if they were too different.
Turns out not to be the case; particularly for microorganisms. DNA is quite promiscuous that way; it gets everywhere. This is compounded with symbiosis.
I think it's wrong to say that the environment is too contaminated with existing life. How would you know that? Species get exstinct and when they do they leave room for other organisms, not to mention the various disasters that have happened during the history of the earth which have erradicated almost all life.
There has never been such a disaster. Some disasters have killed a large majority of animal and plant life- bacteria is prolific and durable. It exists in some of the coldest and hottest places on Earth- in ice, and even under the crust of the planet.
Bacterial life is everywhere. A new life form isn't going to suddenly become a large animal- its competition is bacterial. And in that domain, the competition is overwhelming and ubiquitous.