You're wrong to think that civil action against Knox or Sollecito has little reasonable chance of success because:
It is usually easier to win a civil law case against a guilty party than to convict them.
AK & RS were already found guilty by the courts of merits, and were never found innocent. Obviously this excludes Hellmann, that was however annulled on all reasons.
The Kerchers have the support of final verdicts that provide fact findings (Guede's verdict and Chieffi, plus the incoming second Knox's calunnia).
The defendants AK & RS may invoke the points of law set by Bruno/Marasca, however these points clash against res judicata, which is Chieffi, and are manifestly unusable (you can hardly invoke "sacred protocols" which don't exist in a civil court).
The Bruno/Marasca is not a court of merits, so by definition its conclusions are no conclusion in the merits, they can add no findings to the lower courts ones; they could not make any finding in the merits, but those one that they unwittingly dared to make motu proprio are the placing for sure of Knox inside the house where she could hear Meredith's scream (and we know she didn't die immediately from her wounds) and washed her hands from Meredith's blood. In addition to that, they declared "530.2" that is, no innocence was found.
All the evidence inadmissible in a criminal court, is admissible in a civil trial. That includes Sollecito's changing police versions or the recording of his voice before Matteini, but may include also new evidence, like other testimonies. Even Mignini could be called to testify. The evidence theoretically available is huge, and the burden of proof is lower.
However, a civil action on this matter would take an incredibly long time, many years, it would be very expensive for the lawyers (I mean the costs of the paperwork would be huge for litigators in terms of expenses, not because of lawyers' fee but because of office fees) and in the end, that wouldn't be rewarding on a monetary level, because Knox and Sollecito in the end won't have enough money, that could be easilly taken, to cover expenses and be productive; so, the action is possible, easy to win if the lawyer is expert about the case, but I don't think the Maresca lawfirm is rushing to do it pro bono, because of their obvious expenses. I think they may do it, though, soon or later. Or some other lawyer may do it as well, it's only if the Kerchers decide to. Actually, since Knox & Sollecito won't pay the money, an action against them would be only symbolic. The same judicial truth however, could be established indirectly, through actions against other subjects with more money, which might be a more productive path.