For the most part you’re barking up the right tree. Communication with triffid will eventually be possible, but the route will probably be through researching them and finding out how they communicate, not through mutation. A plant mutant is no more similar to a triffid than a dolphin is to a termite mounds; I don’t think it would give the power to understand their signals and language.
For the most part I think you’ve got the communication stuff pretty right. Of note, triffid are biased towards assuming we’re inferior. Most interactions with them are going to be heftily weighted to the triffid side. However, they are the most comprehensible of the alien types in the game currently, and could be reasoned with along the lines of diplomacy with a powerful foreign nation.