1. Do creatures in the "plants" niche use the Herbivore contest?
Plants are controlled by the same player and one must become a Herbivore if two of them are in the Plant Trophic Level.
2. Wouldn't it be better that the creatures in the Herbivore niche use first the Carnivore Contest and then the Herbivore contest in case there is a carnivore niche filled? (herbivores here could be both hunters and hunted).
The contests are resolved immediately. So if a creeple comes into an occupied carnivore level, a contest immediately endangers one of them. The same if it comes into an occupied Herbivore level, except the loser can trophic shift (which can generate another contest). I hope this answers your question.
3. Raindance - who is Medea in 1, 2 and 3 player Mars/Venus game? Please tell exactly. Which creatures should be more than organs? The Medea card owner?
Per C1d, the player with the lowest skeletal number gets the Medea card to start with. In solitaire games, the single player is always the Medea.
4. If playing a "solitaire" (or 2-3 Venus like game) on Earth, what constitutes a flatland? The seas? What is the highland? Grass and swamps?
Yes and yes.
5. Could we play with automated "plants" but with the usual Megafauna rules? (e.g. normal seas, unmodified events etc.) - I refer to the Solitaire game on Earth itself. If there is an indented variant for this which somehow missed the publication then please show me it.
I did not playtest such a possibility, but would include it in the living rules if you playtest and submit a report.
6. Does the planet freeze if there is no water in the lowlands or both in the lowlands and the offshore "plant fishes"?