"God knows if someone was "validly" (that term is not Orthodox but I will use it for now) baptized before. And they know when we baptize converts we do so out of an abundance of caution. No Orthodox priest is attempting to perform a Mystery that can only be performed once a second time. God understands this and thus no sacrilege is performed."
Our parish has had an amazing influx of converts over the past five years and our practice has been as you have outlined. In one instance we baptized someone who most likely had been baptized a Roman Catholic at birth. This was in the case of a young Filipina who had been adoted by some fundamentalist "free church" type Protestants when she was 4 years old. She of course had no baptism records but we did have a paper saying that her birth mother was a Roman Catholic. I communicated with some Jesuits on the island she came from, but they couldn't find any records. Our Metropolitan decided that lacking any concrete evidence that she had been baptized, we needed to baptize her and if we were wrong, he was quite sure God "will understand".
As for marriages, I think we only recognize Roman Catholic marriages as being fully sacramental and require all other converts to "have their marriage blessed" by The Church. I am also aware of several instances where Eastern Rite Catholic priests have converted and their orders are fully recognized (I don;t know about Latin Rite priests though I expect it is the same. We do not accept Anglican ordinations). There is no re-ordination, merely a vesting. I also understand that that caused a problem for at least one such priest while on Mount Athos.
This, in case it isn't obvious, is clearly the sort of situation in which the Catholic Church would Baptise conditionally. Again, we're not trying to "fool God", much less insult Him by implying that He doesn't know what's what. We're reminding ourselves of the limitations of our own knowledge and reminding ourselves and any witnesses that the Sacrament of Baptism is a one-time-only Sacrament, that it leaves an indelible mark on the soul.
Nitpick: I see that sort of phraseology frequently from Orthodox on this forum. If the term "valid" isn't Orthodox, what term is? Please, let's at least learn to speak each others' language, here. You apparently make a distinction between "a Baptism" and "a ceremony that sort of resembles a Baptism, but isn't one" ... what is the Orthodox term for the latter?