If I count as a Fe user (I'd just say non-Fi user),
1) Do you find yourself easily venting to others about logical problems as well? Do you feel less likely to vent about problems with things than you do when the problem is people?
For me, it's any situation that has me frustrated. That'll often be what happens at work (mostly just commiserating more than venting). Actually, I think (in person) I'm more likely to vent about things than people, because the former doesn't require sharing private, possibly unreasonable thoughts. How I feel about things isn't personal whereas my feelings about people are almost by definition more irrational, emotional etc. And I do know that I have a tendency to judge people more than I should, so I try to avoid airing those thoughts publicly. But maybe the venting about people will be more intense (than about things) when it happens, because I'll restrain myself until holding it in becomes intolerable, and because the situation will usually have more emotions attached. In that case, it will certainly be vented only to trusted people who I don't think will share the information.
2) Many of you said that they're judging the job she did as a mother, not the person itself. What does a judgement of the person as a whole look like for an Fe-user? And how does it come about? How is it experienced? Is it ever personal?
I guess a judgement of the person would tend to be "he's an asshole" or maybe "we don't get along". It tends to happen when the person does things (usually more than a few) that show that they're inconsiderate jerks. Experienced? I don't spend time dwelling on it unless I'm forced to be in that person's company, which I would certainly avoid if possible. I'm not sure what you mean by personal. Of course it's about that individual person.
In this case I would imagine judging the mother personally by saying "she doesn't care about her kids", "she's cruel and abusive", "she's stupid", "she's (optional: white) trash", "she's ungrateful", etc. A few of these were implied by Fi but I'm not sure they were actually meant (she did say IF she cared then...., which is pretty close). Most of what the istp said was descriptive rather than inferring motives, though.

Karma is about how you treat people, not values you hold. People get confused by this and dont understand Karma. It also means sometimes we ARE karma and sometimes we will be judged by someone elses values or basically forced Karma by confused people. They do intertwine(how u treat people and values), but you have to be able to seperate your values from the world and be able to understand that another persons values are different and its not your place to force your values on others and judge them as a person, only judge there interactions based on the part they are interacting with which could be yourself at which point you are judging them based on your values and need to hold them to your values.
As an inferior Fe user I judge people based on how they make the other person feel. Happy is good, hurt is bad. Its simple. Frustrated, confused, angery, etc. are ok and just things to work out and move past.