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complexity + weight probably not worth the trade off. If you can hit it with just main engine gimallinga and grid fins, why bother?

also, combustion devices are always one more point of complex and catastrophic mihaps. better keep it as simple as possible and don't miss.

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But there's flame coming from lower rocket body in the video, that suggests partial RUD around thrust structure, which has to be looked into and worked a lot.

There's already fuel line going into the fins as a ghetto hydraulics, so it should be more of "just" adding the oxidizer line rather than completely redoing that area for something that hasn't been done. Plus it could make the entire stack more flush and sleek.

The engines combust fuel to run their turbopumps, the exhaust will be the flames coming from the side of the rocket body. You can see it on the ascent as well. On Falcon 9 too.

Raptor is a full flow staged combustion engine [0], that means that the preburner exhaust is fed into the main combustion chamber. This is more efficient than dumping it separately like it happens on the Falcon 9 engines [1]. The black, sooty smoke on the first image of the Wikipedia article [1] is the (kerosene) fuel-rich preburner exhaust of the Merlin engine.

[0] https://en.wikipedia.org/wiki/SpaceX_Raptor

[1] https://en.wikipedia.org/wiki/SpaceX_Merlin

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