What is most important for conducting the postmortem of a project, use
a conferencing solutions to perform the postmortem remotely or wait
for two months until the team can meet in person?
Personally, I think meeting people in person breaks down a lot of barriers and empowers each person to contribute to the discussion. In my own experience, I've seen people in video conferencing get distracted or nod off (no joke) during a conference. I'm sure there are plenty of examples out there where this has worked well, I just haven't encountered any.
While the above is more of a critique on remote conferencing vs face-to-face meetings, I feel you could make use of both for your particular problem with the addition of a few other communication mediums that you haven't mentioned.
First, you could have a conference to highlight the key areas you want to address in the post-mortem to keep the team's attention focused. Secondly, you could employ a wiki for each member to contribute thoughts, ideas, or whatever you want them to put forward. When the time comes to have a meeting in person, you can review the content posted on the wiki. It is important that all team members contribute to this as anyone not doing so is liable to forget or take a while to come back up to speed (depending on the size of the problem and their level of involvement).
The second part of your question on how to mitigate these problems can be addressed in a variety of ways. In your example, I've gone under the assumption the project has followed a waterfall development approach and you are now wanting to review the project. If you follow an agile approach, you can have project retrospectives at the end of each sprint to discuss where the project is heading and things you want to improve. I've found retrospectives to be extremely important, but most companies I have worked for don't seem to take them very seriously and communication breaks down bringing my team into a situation not too dissimilar from your own.
A supplementary strategy you could employ is to make use of a wiki. Create a post-mortem section for the project and ensure all team members contribute to it and review it.
Remote teamwork is all too common and it's very easy for people to forget there are other people in the same team, working remotely, which is why face-to-face meetings are important to put faces to names and break down barriers. What I believe you want to do is encourage communication within a team by give them as many communication mediums as possible.