First PR vs. single member.  Both systems have their advantages and disadvantages as to democratic legitimacy.  In the former you get a better representation of the parties the voters want and you don't get situations where multiple parties give you winners with well under 50% of the vote.  However, all is not perfect in PR. PR tends to create coalition governments where small but crucial minority parties get outsized power and influence since the big parties have to get their support - classic examples are the FDP in Germany and the religious parties in Israel. The alternative is a grand coalition type situation like you currently have in Israel and might well have in Germany.  Furthermore, a PR system gives a huge amount of power to the unelected internal party leadership.  It is very dangerous to go against your party leaders in a PR system since they can make sure you will lose your seat in the next election, regardless of what the voters might think of your actions.  

As to getting some of the far left groups to start cooperating with the moderates - not realistic. The two groups strongly dislike each other, and for good reason.  People like myself often find the mainstream right preferable to the extreme left.  The extreme left on the other hand has made it clear it prefers allowing the right to win rather than seeing social liberals in power.  

But, just for the record, this is what it would take for me in Germany. A transformation of the PDS so that the majority of its functionaries and activists are no longer ex-SED - in other words, get rid of most of the people currently in it. In the West, get rid of Lafontaine and anybody else who is basically NPD lite.

In France - get rid of people who think that full blown socialism needs to be established, and anyone who likes the idea of a sudden radical transformation of society - the grand soir types. Get rid of the xenophobia.