It can be easy to understand where they would say that your posts would come off one sided because they are one sided. You do not know their exact feelings, but when you are in a relationship with someone, you can somewhat gauge what they are feeling. I have read all of your posts before replying to this because I know that just reading one post can give you a misleading judgment of the author, but all of your posts say the same thing, which is woe-is-me. I can tell that there is a part of you that knows their relationship is hanging by a thread and that if their relationship faulters, then so could your entire life, but you come off like you only care about what they could do for your and what is wrong in your life. They probably have many issues they are working through just as you are. You have said that you are closed off and that would make you hard to deal with and a hard person to be in a relationship with. Maybe looking at your post was a way to try to break the ice and find something to build off of if you weren't giving them anything to go off of. I can see where they would say that you demand attention because you have posted many of the same posts, just in different ways. You need to spend less time on this forum looking for insight from complete strangers, and more time opening up to them and talking with them about what is going on.
In regards to what you are asking advice on:
1. Sam's jealousy: ask her why she is jealous and see if you can work with her to get passed it, but understand that jealousy is a strong emotion and it will take time and effort
2. Glenn's distrust: he has been hurt and will of course be leary of anyone hanging around you. You said he is accusatory of you having feelings for your friends, try taking a look at how you act around these friends and see if his worries have any bearings.
3. Being leftout: You can into an existing relationship and they will have things in their past that they will be proud of and want to celebrate and you have to understand that. I am sure you do things that make them feel left out too, as you have mentioned with Sam's jealousy.
4. The baby issue: you are young and can have more chances to have children. If you are this concerned about the status of your relationship and can't find anything but problems, then you shouldn't want to bring a child into it now anyway.
5. Sam's infidelity: you have said you would give them time to work on their problems and seem to be angry that they are trying to. Also, you said "trying to fix his marriage", when isn't it their marriage, again making it seem like you put more priority into him than her.
6. Quality time: know that everyone in the relationship needs individual, couple, and poly time and try to plan a schedule accordingly.