Here is a completely non-partisan, logical consideration of your point
1) There is already a pipeline that runs from Canada to Port Arthur, Tx. - correct, but your own point 4 undercuts the notion that it is sufficient - running oil through train cars is significantly more pollution (you have to run the train after all) and the accelerant to the Keystone XL pipeline was that a train derailed and spilled all over a nature reservation. It was then found out that more pipeline would have avoided all that.
2) Farmers, ranchers, American Indians, all that would stand to make profit from the pipeline, most, do not support it. Environmentalists are against it. Underground water supply could be polluted. As much as we like and need oil, we can't drink it.
Farmers are always against taking their land under executive order as the land grab never ends. I concur with their issues but there is a balance to be struck somewhere. The history of the "American Indian representatives" is full of anti-government sentiment stoked by white people more interested in protecting the idea of 'the noble savage' which left their populations in the state they currently are in.
As far as the groundwater pollution, that is assuming the pipe leaks non-stop without anyone fixing it. Pipelines don't just leach things into the ground, everything going in the ground is waste and a potential for expensive cleanups. Hence what I said prior, engineers have figured out ways to build pipelines without leaks and plenty of backups.
3) Only 48% of Canadians support this 2nd pipeline(XL). This could mean that 52% are against or don't have an opinion on it.
Or it means the media has painted a bad picture of the thing. You could cut the same to say only 30% of Americans support Biden because the rest either voted Trump or didn't vote. It's a bad argument if you don't present all the pros-and-cons honestly.
4) Canada is on track to set record oil exports to the U.S via railcar and the existing Keystone pipeline that has been operational for years now.
See point 1 - there are seriously more issues with railcars than pipelines
5) Many of the jobs related to the construction of the pipeline are temporary.
That is a bit of a tautology, because the pipeline won't be endlessly constructed. So yes, those jobs are temporary, but there will be more jobs at the refineries and transportation of finished products, the trucks and boats and trains that it fuels that drive the economy etc.
6) The oil that would run through XL pipeline is a thicker corrosive crude that is extracted from the Canadian and Nebraska sands, that creates more pollution than normal oil operations.
According to whom? And yes, pollution happens when we extract and burn oil. Low-pressure collection of shale and fracked gas/oil is cheaper and thus less polluting than drilling high-pressure holes far off-shore and deep into the ocean floor (remember the BP leak a few years ago that took months to close and Exxon-Valdez for the transport of said oil). But the only true zero-emission (nuclear) has been paused indefinitely by the same environmentalist and wind/solar is both too unreliable and too expensive and very, very polluting (heavy metals, lead, rare earths - look at factories and mining in China that produce solar panels and large motors - they are slowly poisoning entire cities with all their major cities now covered in a fine yellow haze from the byproducts of heavy metals like lithium, which is worse than the smog we had in the 80s and 90s in the West). There is a balance between dumping the oil in the nature parks and using 100% of it without emission. A pipeline is the cleanest, least amount of energy required to transport oil, regular transport has major issues and is a lot more expensive, which is why they want a pipeline in the first place, to make cheaper and more profitable oil.
