Wow! A ressusicated thread!
Charliehall is absolutely right: so much so that the gemoro itself paskens ‘mamzer vadai omanr rachmono velo mamzer sofek” In other words, the essence of a psak on “mamzeirus’ needs absolute certainty. Otherwise, ipso facto, he/she is not a mamzer.
And yet, nisht,there is n osuch psak on agunos but Poskim have always looked for any kuloh.
May I also add that, in similar circumstances, there should be kulos with geirus too- in line with the Rambam and many Poskim-and in contrast to the ridiculous Piskei halocho on geirus by recent Israeli rabbonim that made no sense. (psul lemafrea?)