Decomposition products of PET
This is a partial answer to your query.  According to the paper Degradation and Recyclability 
of Poly (Ethylene Terephthalate) 
 (Venkatachalam, 2012), there are a few degradation products, from the article:


  Apart from aldehyde, $\ce{CO}$, $\ce{CO2}$, ethylene, benzene, biphenyl are also identified as 
  degradation products 


(The article also shows diagrms of the chemical processes of PTE degradation).

an important aspect to note is experiments/observations have


  shown that rate of thermal degradation in the closed system is 
  about three times higher than that in open system.


In regards to burning of PET, the products can vary according to the page EPB 433- Health and Environmental Effects of Burning Waste 
Plastics 
 (Saskatchewan Ministry of Environment) stating that


  the composition of byproducts of plastic combustion as to their type and concentration depends 
  on the combustion temperature and the flame residence time. 


According to the Peiyu Plastics page (a reference from Wikipedia), if complete combustion occurs, then the products would be $\ce{CO2}$ and $\ce{H2O}$, incomplete combustion could produce soot and carbon monoxide.

Further reference:

Here is the MSDS for PET

A study of thermal decomposition and combustion products 
of disposable polyethylene terephthalate (PET) plastic using 
the High Resolution Fourier Transform Infrared 
Spectroscopy, Selected Ion flow Tube Mass Spectrometry 
and Gas Chromatography Mass Spectrometry  (Sonova et al. 2008).