Tarantino actually wrote FDTD way back in the late 80's/early 90's as a favor to Bob Kurtzman from KNB FX. I believe Tarantino was paid like $1,500, plus Kurtzman agreed to do the effects for "Reservoir Dogs" in return.
As to why the movie gets crappy toward the end, I don't know. A lot of the silly stuff kinda ruined it for me too. Although I have to admit that some of the one liners are the best Tarantino's written.
"You're dealin' with a coupla real mean motor-scooters."
"...I will turn this place into the fucking 'Wild Bunch' if I think that you are fucking with me."
"Don't you ever try and fucking run on us, because I got six little friends and they can all run faster than you can."
And, of course, so good that we included it in one of my band's songs...
"Kill 'em ALL!"
Etc., etc., etc.
As to why Tarantino would even bother acting in the movie...Q.T. has this misconception that he's a decent actor. If I recall, there was a heated debate around Hollywood as to whether or not the man actuallly COULD act. I suspect he took the role to bring an end to all the trash-talking. Did it work? No. People still say he's a shitty actor. I've posted here before that he's no DeNiro, but he's definitely better than Keanu Reeves or Vin Disel, but I digress.
Anyway, some of your points DO beg rational answers. I did my best to shed what little light I could. Hope it helped. Later.