It does open up later. After you rescue the Turian general I think. You gain access to the Presidium commons a night club and a refugee Camp.
Spoiler
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Kelly Chambers, from ME2, is in the refugee camp. you need to tell her to change her name or something unfortunate happens to her
You can only get rid of the medigel after the citadel opens up. there will be a new female doctor in the 1st room of the hospital. A tip is to bring up the map and then you can scroll through all the areas on the citadel and it will tell you where people are.
Of course once the citadel opens up you start picking up more assorted crap that you can't get rid of until another event happens at the citadel.
You can also lose missions by not doing them in the right order. I have read that only the Priority missions advance the story so as long as you do everything available before you do them you should not miss anything. Its worked for me so far.
Go here for a list of all the missions for all Mass Effect games. scroll down for ME3
masseffect.wikia.com/wiki/Missions
And yes as Glass Kop says there is a real shit storm going on at the official forums over the endings.