High oxalate foods are primarily a problem if you are one of the unlucky peeps to have wiped out the oxalobactor in your gut with antibiotics. I can't remember off the top of my head how many adults are affected by this, it is a significant percentage, but less than half of the population. If you have been on many rounds of antibiotics in your life perhaps exercise caution, but it you've already been doing this for a long time with no problems, you likely have the right bacteria to deal with it.

Cellulose, cooked or raw, is indigestible by humans. We aren't even particularly blessed with cellulose-digesting gut microbes. By cooking greens you are breaking down the cellular matrix making them easier to digest and the nutrients more bioavailable. You're likely to use fat as well, which aids in absorption of fat-soluble nutrients.

Oxalates are an anti-nutrient boogeyman that I don't think deserves so much disdain. Like phytates, they can bind minerals, and like phytates, are found in most plants. I think you'd be hard-pressed to eat enough greens (except maybe sorrel) to have issues.

And if you have a leaky gut... then oxalates will travel out of your stomach into your blood system and can cause other medical issues (autoimmune related). Spinach and red beets are the biggest high oxalates offenders followed by tomatoes, the rest on your listed are low oxalates.

I found good info here: http://www.whfoods.com/genpage.php?tname=george&dbid=48.

I had some issues with high oxalate foods, cutting back spinach and replacing it with other greens is a good first step. Also, reducing carrots and beets seemed to help.