In The Groove FAITH edit got tempo bugs and no double mode

Some of the songs provided by this italian stepmaniac called Faith (especially those contained in Dance Pack 7) have annoying tempo bugs (step beat slightly shifted above or below song tempo) making it impossible to get 100% even on easy modes.

Also, there's no such thing as Double mode (the only one I actually enjoy playing)

I don't even know if it's legal to incorporate commercial songs such as Whistle and Wrecking Ball in a game like In The Groove.

For now, I will stick to the good old songs who actually belong to In The Groove, such as Kiss Me, Mongolian Grill and Vifta Med Handerna.

I'm not sure what you're saying. Random charts could have bugs and pretty often don't have double charts.

It's not legal to incorporate songs like that, but that's kind of the modus operandi of the ITG and the larger StepMania community. I'm not entirely sure if Mongolian Grill, for example was used legally, in fact I'd be surprised if it was. Vifta Med Haderna is a Basshunter song, and thus was almost certainly used illegally. And neither of them really are part of ITG either, though Kiss Me definitely was an actual ITG song.