Indeed being single can impact a person's iman. The single sister who had no responsibilities except those she chooses is in a better position to focus more on her iman and growing spiritually; whereas the single sister who has other responsibilities (job and running a household and raising children) is forced to focus more on these worldly things for the sake of actual survival and therefore has less time and energy for strengthening herself spiritually no matter how much she wants and needs to. Yet these are the same sisters who would benefit from marriage and are the ones who are more often rejected except by the brothers that they are totally incompatible with.
Yes, singlehood can impact a person's iman. So does being married. And so does being a parent etc.
Yet, I deeply believe that Allah, The Most Just would not cause one group to be disadvantaged spiritually over another group just because He willed more challenges to surface in the former's life.
I think apart from the exceptions, most single sisters would benefit from marriage, regardless of their status and challenges. If what you mean is that single mothers would benefit more than those who are never married, then in our limited perspective, yes, they could. And so would older, devout sisters over younger, but ignorant ones. These are priorities that society should place upon, but as you said, the reality beholds otherwise.
To counter the product of messy social engineering or whatever we call it, is to make best of what Allah has willed upon us. If ALlah has not willed marriage on us, that is not justification for us to swim in heedlessness or a perpetual excuse for a weak iman. Yes keeping up with our iman may be harder, less time for zikr, less time for this and that. But that is living - a continous struggle. And marriage per se would not miraculously cause one to be in higher station of closeness to Allah if one's singlehood is spent in the other direction.
The suggestion that sisters need to do a lot of reflecting on "did they find a brother whose deen they were satisfied with but reject him anyway" isn't entirely fair because part of sound deen is being able to be financially responsible so if you find a brother who can do all those requisite Islamic things but he's unemployed and broke how could he be a viable choice at that moment?
My statement was not said in a vacumn. Certainly there are "disclaimers", "pre-requisites", "context" etc. And when I said "deen", I do mean "deen" in its comprehensive meaning, and not just in one who is seems devout in fulfilling the rituals of Islam.
And that's where reflection comes into play.
My post is not to blame, but for us to find a way out. For our hearts to be disturbed by the corruption that human beings have caused, but yet, be at peace with what Allah has willed.