I really don't have an answer to most of your questions. I will have to say that while Gladiator was entertaining, it was far from historically accurate, although in fairness it did not claim to portray historical events. One of the things that I did like about gladiator was the opening battle scene; IMHO it was of the depictions of the Roman Legions in battle that I have ever seen.

Having grown up in Italy, and having actually gone to Italian schools, I was always taught that SPQR was an acronym that meant Senate and Populace of Rome (Senatus Populus Que Romanus); I hope this helps.