1. The only party calling for a united left front is SYRIZA, and they have already started cooperating with various left factions that have abandoned the Socialists when they brought in the IMF, on both a grassroots and a central level. The Communist Party however (without which as the numbers stand today there can't be a left government) is adamant about not collaborating with the reformists who want to beautify capitalism (I kid you not), and are rejecting any collaboration with SYRIZA even at a grass roots level. If that doesn't make sense to you, its because it doesn't make sense generally. This is a party that organizes its own separate demonstrations even when protesting the Israeli bombardment of Gaza. It recently attacked the Communist Party of Spain as being capitalist tools for participating in IU and the Party of the European Left. The only thing that might change their mind is massive pressure from their base, which, though happening at some level, hasn't been strong enough to move them yet.
The Democratic Left meanwhile seems more interested in talking with PASOK cadres and ministers than with their former comrades. It remains temperately anti-troika and anti-austerity, though it is much more likely to compromise with the powers that be, than form a part of a left-radical solution.
2. Even if the issue of collaboration between the left in Greece was somehow resolved (either by the strengthening of one pole, or the popular pressure on all of them), it is not at all a given that elections would not be postponed. The current parliament has been elected for 4 years, until autumn 2013 that is, and despite the conservatives having made a deal that elections will be held March-April at the latest, most of the elites don't like the idea and the conservatives themselves might have a change of heart should they begin to have doubts that they will be the first party come election day. In that case I can easily imagine some sort of "emergency" producing an agreement between the three coalition partners (socialists, conservatives and the far-right) to postpone the expiration of the Papadimos government until the end of the parliamentary period, late 2013, and beyond.

The road of excess leads to the palace of wisdom - William Blake