"Life force energy is drained" Doesn't support the thermal energy idea, nor the water idea.
"With life energy draining, things get cold." It's hard to tell because of the translation, but "Plants, animals, everything is getting cold" could be referring to their experience as life, and readings of life around them. This seems to imply the "life energy drain" interpretation, or at least that that is something that's being drained. Thermal energy alone wouldn't make sense here as things are getting cold instantly, thermal energy wouldn't instantly make everything cold. It also doesn't seem to imply the water interpretation either, as everything's getting cold and freezing over (which requires water) rather than getting dry.
"The planet would be drained of all heat energy dry." The translation there actually says "They're planning on sucking this planet dry of energy". "of energy" being the important part, things can be sucked dry of some things without being sucked dry of water. This is neutral to the thermal/life/water interpretations.
"It is stated again that all the planet's energy will be drained." Another statement that doesn't explicitly lend credence to any interpretation, but it does make me wonder if this planet has some ethereal special energy, separate from thermal, life, water, etc. but some inherent spiritual energy to planets, as this planet's energy is described as having "exceptionally high purity".
"As life energy drains, flowers and leaves wither into dust." This scene seems to imply the thermal energy interpretation as false, as rather than freezing due to lack of external heat, they wither away due to internally not having enough energy to sustain themselves.
"The machine is shown to be draining the energy from surface lava vents." Really? At least in that single cutscene I didn't really see that. The machine was constructed on surface lava vents (which weren't drained for some reason), and the draining particle effect isn't in the air, and the cutscene doesn't have explicit shots or mentions of it draining lava, so maybe I just missed that part as it flew by in the background. But the mere existence of lava shows that it couldn't have drained 100% of the planet's thermal energy, meaning the exact amount it has is uncalculable, and should be lowballed as 0.
"The destructive force this planet's energy can bring forth cannot be measured!" There are exaggerated statements all over fiction, that's why we almost always go with demonstrated feats and not statements.
"Energy returns to the planet." I already discussed earlier, but mentioning again that the energy making frozen land go away implies that it couldn't have absorbed water.
Overall, in my opinion, both the "thermal energy" interpretation and "water energy" interpretation have multiple cutscenes blatantly contradicting them, with only a few cutscenes implying them. The energy being drained being the innate life energy of the creatures, or being some innate spiritual energy of the planet itself, seems to be the least contradictory things, although both of those have their own minor issues (why are Sonic and crew not affected while the rest of the life on the planet are?), however that issue also applies equally to the other interpretation.
@Weekly I'd still like to have some discussion over this, but I can understand if you don't want it cluttering up the thread. I at least find it interesting and worthwhile to clear up such a misconception if some people would use it as backup for High 6-A sonic being consistent.