No, I do not speak french. Unless you count any french word that commonly appears as a musical instruction.
Clearly, the guitar and bass were detuned without telling Janne. He seems to have caught on pretty fast. Alexi is having issues singing down a half step (half steps are hard- they can really trick the ear- it would have been easier on everyone to detune by a whole step or a consonant interval like a third or fifth but that would really be hard on the string especially if it's not brand new and high quality which I'm sure they are). Detuning a string has an effect on the harmonic series (aka the overtone series), and that is where the difference in tone arises between a string tuned as designed and a string that is detuned. Typically when detuning a cheap or overly stretched string it will vibrate a wider pitch range for each single note (generally it is preferred to keep the vibrations within 5 semi-tones- an easily noticable pitch difference is 10-20). I have heard that Alexi's signature strings are not preferred by musicians that habitually detune. I have not tested this. It is likely that his strings are designed for a specific detunement that most musicians who detune are not using. If you do like to detune, you should select a string endorsed by a musician who regularly detunes the interval you wish to detune. Honestly, the only reason for a rock musician to buy really expensive strings is to detune. Cheap strings will wear out faster (especially if you detune them) and need more frequent tuning, but new they will hold pitch fine. If you are just learning, it is actually preferable for you to use a cheap string as the need to tune it more frequently is good practice. Conversly, if you are just learning to detune you should put a lot of consideration and a little more money into your string choice.
I would be an ass and tell you that I can hear these are clearly Mick Thompson's preferred string, but it's just not reasonable to hear such a difference. Now, when it comes to the different materials used to make the string it is much easier. It is reasonable to expect a person to easily hear a difference between a steel string wrapped in brass and a gold string. Or perhaps a string wrapped in copper (would be easy enough to tell when it turns your fingers green right?)
Rhythmically, it is more exact. I'm sure that Janne started playing the chorus to help Alexi sing the transposition. Supporting harmony is an OK way to say it- usually we just say harmony. You might be talking about a counterpoint harmony. I'm not sure. A contrapuntal harmony differs from the usual harmony in that a typical harmony is just chords while counterpoint is transposition of the main line (both harmonically and rhythmically).