If it is in fact 'chemically identical' to crude oil, does this mean it can be used to produce plastics etc? This would at least be useful and require a much smaller scale of operation, rather than simply wasting it by burning it as usual.

I'm sure you can. Wikipedia has a long entry on bioplastics:

http://en.wikipedia.org/wiki/Bioplastic

My educated guess is that this algae oil is like extracting potash from seawater, theoretically feasible, but no one is yet making a profit from it.

A plethora of small cap start up companies have been working on many solutions to the energy crisis, few of them will ever make a profit for their investors, nor do they provide affordable energy products. A few companies might do well amongst many companies destined to fail. Most new businesses do not make it.