Wellinformed, you are really not well informed on this issue and you are not being realistic. You think that just because a person has a past he will marry someone with a past? But you just said that your posek paskened the past should remain there and he doesn’t have to divulge it, therefor he nor she will be looking for shiduchim that have the same background. Once they have “returned” they do not have to swim in that pool any longer as far as you and your posek are concerned. And just because an individual does not wish to remember or deal with their past any longer because they have b”h reached a higher madreigah that doesn’t mean that their acquaintances from the past whether they have improved or not can not be relied on to not say hello and not approach them with in joy in seeing their improvements or just to say hello. So that concept of yours is false and does not hold water. You cannot stop another person from approaching you.
And as far as what marriage is about, yes it is about the future but is also based on a foundation of honesty and trust not secrecy. If you believe that you can build a marriage on secrets then you are totally mistaken. A marriage built on secrets is doomed for failure. If this is the advice of your posek who deals in marriage counseling I wouldn’t trust him with any couple I know. He obviously does not have the appropriate training to counsel anyone. Couples who need counseling should go to trained counselors and not Rabbonim who dabble in marriage counselors. Rabbonim who concentrate their kochos in Shalom Bayis is another parsha altogether.