One other reason why I don't think it will be as big a problem as you think it will:

I think it may be that your perspective has been skewed by your having gotten into many flame wars with Obama supporters in the Fray before. Flame wars are often nasty, and you may therefore perceive far more nastiness from Obama supporters than the typical HRC supporter perceives. As a result, you may be overstimating the degree to which perceived nastiness of a candidate's online supporters influences his opponent's supporters to resist party-unification after a nomination has been decided. In actuality, it is only a minority subset of HRC supporters who would be influenced in this way.

Most people buy into the meme (widely circulated in the MSM) that the differences between HRC and BHO are more stylistic than substantive. HRC supporters who agree with that meme may simply find her the better of two good options, but they still like both. For them, it won't be hard to settle for their second-favorite candidate.