You've identified the key issues about lighting and context. My gut response is that a 2.x is going to give you more of what you want. The Tamron 150-600 is wonderful lens, though, and probably goes on your wish list. In the meanwhile a tele will look great. I think a lot of the critiques against tele's is grounded in older technology. Yes, it's not going to be super sharp as a monster prime--but within limitations, it's still going to deliver amazing images.
This image was taken with a Sigma 70-200 with a 2.x -- fully out to 400mm. It's f8 at 800 ISO and 1/350. I also used a polarizer (which requires another stop, but at the equator, bright sunlight is a true nuclear force).
You might need to clean up some fringing in PP, but as long as you are in good light, with a decent f/stop and reasonable ISO, you'll be good.
For the range your lens covers, on a crop sensor, it delivers very respectable resolution.
I recommend the normal steps for stability, though, as extenders will throw off your balance on a tripod, if you don't have a tripod collar.
For a twist on remote triggering, take a look at the DSLR Controller app. Very powerful.
6D / Canon 35 f2 IS USM / Tamron 28-75mm 2.8 / EF-S 55-250mm / Sigma 70-200 2.8 / Sigma Tele 2.x / Samyang 14mm / Yong Nuo flashes / Orlit Strobes Pixma Pro-100 / Epson P800 / ColorHug / Tokina 100mm AT-X M100 AF PRO D / Linux Ubuntu / PrintFab-TurboPrint / AfterShot Pro / GIMP / DarkTable / Zerene Stacker