khl, The Torah Temimah was a talmid chochom, but not an Odom Godol. His opinions were not considered as coming from a Daas Torah, though he was well respected for his knowledge. His derech halimud and Hashkofo were influenced by untraditional sources and he sometimes said things (some printed in the Torah Temimah) that may not be said. He was resepected as a highly knowledgable person but not beyond that level.
The Maskilim were very invovled in “correcting” our texts and changing Girsaos that they werent big enough Lamdonim to understand, which happened a lot. The Torah Temimah does this at times too – prefers to change Girsaos where more acceptable solutions are available. His most famous instance is not in the changing of a Girsah but rather his statement that the Makas Choshch in Mitzrayim was not darkness of all, but rather cataracts. Whatever. The Torah Temimah is a very useful sefer, but because he was not a Torah authority, we take what he says with a grain of salt, or, as in the case of the cataracts, we dont take it serisouly at all.
As far as his historical recollections, they, too, are not considered authoritative. Its not necessarily an issue of lying, but rather reliablitiy and accuracy, and his seeing things through his own personal perspective.