Why did this thread turn into a debate regarding whether Mr. & Mrs. Smith can really "get it" regarding seat-saving since she doesn't have kids and asserting/strongly implying her opinions will change when she has a child (which seems like a mean way to comment on someone's pregnancy - to offer a back-handed compliment)? IMO, that's unkind and unnecessary. I really dislike it when people tend to target on one person in a thread - especially when it's over a secondary issue not directly related to the reason for the post - such as whether someone's opinions about handling toddlers' and children's needs will change after having kids. Again, IMO, there have been a handful of posters who have done this, and I just don't see it as helpful or informative on Theme Park Attractions and Strategies. Maybe on the community board where people shoot to kill

, but not here. This thread is about seat-saving, personal space, and whether or not you should have to move if you're on the end. Whether or not the people that have posted their opinions on these issues have produced offspring seems highly irrelevant. I know some people won't like this post, but seriously. Enough with the commentary on people's status as parents.
Here's my opinions to the original question:
1) We don't often make use of seat-saving but will if a) one or two people in our party are getting ready to hit the ride vehicle in a ride queue and texted/called us that they'll be there soon (as in minutes), b) someone has to go to the bathroom, or c) someone runs to a cart to get a drink. Other than that, no seat-saving for us. And I don't subscribe to the belief that we should save seats just because.
2) I wouldn't use bags to save seats. If, as I mentioned in #1, someone ran out quickly or will be there soon, I'd just try to keep their space open without putting some object there. However, if someone came up and needed that spot or a CM asked me to move in, I'd do it.
3) Little to none on the personal space front. We usually are within 3 or 4 inches of the people next to us at shows that don't have individual seats (F!, Spectro, etc.). Them's the breaks.
4) YES! Fill in all available spaces. I see how this can (and sometimes does) contradict my opinion on saving spaces, but I reiterate that I think saving one or two spaces for someone who dashes out or will arrive in 5-10 minutes is reasonable. I know others will disagree. My opinion isn't so important that I'll fight with someone who challenges it in the park because, in reality, this has never been a problem in our trips. Either we go at slower times (although we don't) or people around us happen to be easy-going. The most frequent situation like this that we encounter are those people who sit in the middle of a row at PhilharMagic, and because we choose to follow the rules, we step over them (and sometimes on them, which I'm sure is unpleasant and I don't like doing but can't be helped). We also manage, apparently, to hydrate, visit the facilities, feed ourselves, and get in line together 99% of the time. I can think of one time when we didn't in the last 8 visits in 8 years, and that was a situation in which three of us were coming in to F! after having ridden ToT. We got there with 15 minutes to spare and there were still empty seats, so I don't think we denied anyone anything. If we did, I apologize, and if someone politely points out that they need a seat and it appears I have a few free spaces around me, I'd agree and - as I said - move over and tell my missing family member what happened by phone or text, or I'd give up my seats so we could be together.
Oh, and I have no children. I do travel, every trip, with my now-3 year old niece who is recently potty-trained, is strong-willed, has been known to scream bloody murder when she doesn't get her way, and is a positively fabulous human being.

We try to keep her occupied while we wait for things. IMO, this doesn't make me better or worse than any of the rest of you - seat-savers, non-seat-savers, waiters-with-toddlers/kids, non-waiters-with-toddlers/kids,
scooter-over-ers, non-scooter-over-ers, etc. I just want us all to get along and stick to the topic at hand. These remarks constitute my opinion and touring philosophy, which has apparently worked as I've only chastised one fellow guest at Disney (but that was about a HA issue).