I'm not sure I fully understand all of your points, but there are several studies suggesting screen-time i s linked to metabolic syndrome regardless of physical activity; here is one study:
http://jpubhealth.oxfordjournals.org/content/30/2/153.short

If you read my other posts on screens & metabolic syndrome, screens & the stress response, and screens & obesity you'll find numerous references at end of each article:

http://www.psychologytoday.com/blog/mental-wealth/201312/overlooked-factor-in-the-childhood-obesity-epidemic

http://www.psychologytoday.com/blog/mental-wealth/201301/teens-screens-and-metabolic-syndrome

http://www.psychologytoday.com/blog/mental-wealth/201211/screens-and-the-stress-response

And yes, completely agree that sitting and eating sugar rich foods are part of the equation that create vicious cycles. But the screen itself also induces various physiological responses from the subcellular and molecular levels to the hormonal and organ system levels.

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