Oomis, I think the reason a Y”K is not mechaper on BA”LCH without mechilah is because the person hasn’t fixed the aveirah; it’s like “toivel v’sheretz b’yado”. In fact, at times, the offended is obligated to give mechilah, and the offender, having done his due diligence in [sincerely] attempting to appease, can get a kaparah from HKB”H even without mechilah from chavero. I don’t think it has anything to do with which is more chamur per se.
Surely, though, a lack of BA”LCH, even in someone who seems to display exemplary BA”LM, indicates that his BA”LM is false. I think the reverse is true to some extent as well; the ultimate bein adam lachavero is performed as part of one’s avodas Hashem, which is only sincere if it includes bein adam laMakom (assuming that it’s not deficient merely because of a lack of knowledge).
It’s definitely more pleasant to deal with someone who excels only in BA”LCH than with someone who excels only in BA”LM, but both are severely deficient.