Perfect example.

"Until proven otherwise, deforestation and fossil fuel consumption are solely human activities."

In an exact scientific meaning, the quantity of the total CO2 caused by all deforestation and fossil fuel consumption per year is smaller than the error of the estimate of the total quantity of CO2 in the earth's atmosphere. And since one hasn't quantified the amount CO2 in the atmosphere to an accuracy of say 1 billion tonnes, it as scientific accurate to say that total quantity from all the deforestation and fossil fuel consumption is somewhat insignificant.
And what that means is that when I say there is about 2930 Gt of CO2 in the atmosphere it's an approximation, it means it possible that there could be 3000 Gt or less than 2900 Gt. It wouldn't wrong to say there was about 3000 Gt or say there was about 2900 Gt. I accept the possibility that the number of 2930, 2900 3000 Gt either of them or all of them could off 100 Gt or more. I don't know which of those numbers is actually closer to the actual quantity if accurately measured and it's possible that someone actually spent more than few minutes, perhaps a few hours or days, and arrived at a more accurate number.

With all that said, there it has been claimed that about 26.4 Gt human emission per year. Here: http://www.skepticalscience.com/human-co2-smaller-than-natural-emissions.htm

Now, the number 26.4 Gt doesn't give me much confidence that it's a accurate number because I don't accept that they have measured all human emission that precisely. I seen much smaller estimates and I believe 36 Gt is sometimes given. A more precise number would probably be "20 to 30 Gt" or 20 or 30 Gt. I would go as far to say that the 26.4 could seen as fraudulent, because of giving the number to a tenth, it suggests a certain degree of accuracy. But they probably just added a column of somewhat accurate approximation of various human emissions which were given in tenths or even hundredth or thousandths of Gt and rounded up to .4.

Anyhow, it's probably fairly safe to say human CO2 emission is more than 20 Gt. To article Raoul linked:

"...says today in the journal Science that tropical deforestation releases 1.5 billion tons of carbon each year into the atmosphere.

"Deforestation in the tropics accounts for nearly 20 per cent of carbon emissions due to human activities," Dr. Canadell says."

So, if Human emission was 7.5 Gt, then 20 percent of this would be 1.5 billion. If emission was 20 Gt then 1.5 billion per year would be about 7 to 8 %. If human emission was 26.4 Gt it would about 5 to 6 %. Maybe Dr. Canadell is correct. But if Dr. Canadell is correct, then sort of indicates how insignificant 1.5 billions tonnes is if other estimates are so far off.

As for: "Until proven otherwise, deforestation and fossil fuel consumption are solely human activities."

Trees like any living thing dies, so if one a bunch of trees planted at around the same time, they could all die around the same time- hence "deforestation", but as trees die, other tree generally grow in their place. But no place is has deforestation for any significant length of time- even cities aren't immune if they were abandoned- not to mention there are generally lot trees in most cities.
But probably the most severe deforestation occurs in "some" forest fires, the type normally called great forest fires can create quite a bit of complete and utter deforestation of the scale never accomplished by humans logging anywhere. And then you can trees which get diseases and bugs.
As far as fossil fuel consumption- it's possible humans learned about fire from fossil fuel *naturally* burning. It can occur. Granted, lightning is far more common. There coal fires burning all over the world- most in China, they may have all have been started by humans activity [I don't know], but humans don't want them to be burning.
And generally if coal or other fossil fuels are left for a long enough time, they can have an opportuniuty to naturally burn, because geologically earth is active, land is subducted, and lifted, there are volcancos, there is lightning or whatever. What has been around for around billion years is oxygen in the atmosphere which can react with the fossil fuels.

I'll comment on rest of your post, but the moment I got to do something else.