i would like to know -- if i group a pair of One SL with a Playbar (not as surrounds), will i be able to use Sonos 2 app for the group?
YES
Is the Playbar become AirPlay2 because it is grouped with One SL that are Airplay2?
YES and NO. The Playbar is participating as a AirPlay2 speaker because it is grouped to the One SL’s from whence Airplay2 was started. The Playbar itself is still not Airplay2 capable (i.e. able to be selected as an Airplay2 speaker).
When it will be time to watch TV, is the pair of One SL that are always grouped will play automatically or only the Playbar will play by default?
The grouped One SL’s will play the TV audio however with a 75ms delay. The delay may not matter is the One SL’s are in another room away from the Playbar. If they are in the same room you’ll have an echo effect.
What i was told is that when one item of a group is Gen2, the entire group will play as a Gen2.
Is it right?
Someone has their wires crossed. 
They were probably referring to the S2 vs S1 Sonos App and products that are compatible with S1 only vs S2 only vs those compatible with S1 and S2
Click this link to learn which products are S1, S2 or S1/S2 compatible.
To further clarify Gen2 products vs Gen1 products (of the same family) has nothing to do with playback. For example a Sonos One (Gen1) and Sonos One (Gen2) can be paired to create a stereo pair. The only difference is that the Sonos One (Gen2) has more memory for future feature updates. That said the Sonos One (Gen1) may someday be left behind as far as features are concerned when used as a stand-alone speaker. However, the lack of memory will most likely not prevent the Sonos One (Gen1) from being paired with a Sonos One (Gen2) as a stereo pair.
Thanks for the quite detailed answer.
For the 75ms delay, is it 100% sure that it will be the case or it depends of the wifi strenght and speed in the room?
75ms is the minimum latency. In very difficult wireless situations a longer latency is available as an option for Line-In sources and may be helpful.
