Need advice-prefer from a mechanic but all welcome!!

Recently had my timing belt and head gasket replaced. I went to these guys because i
had just put alot into my car prior to this and they allowed me to make a payment plan.
a business that comes to your location. (bad idea) The guys said it would take 3-4
days and looked my car over. The next day they asked if i wanted them to do the
work. I said yes and they said they needed the parts money up front. Gave them the
money and they sent it off to the machine shop. a week or two later they started to
work on my car (after scheduling and rescheduling with me) they said the machine
shop damaged the part and had to order a new one. more time went by and then they
recieved the wrong part and ordered a new one. more time went by. schedule and
reschedule. at this point i considered going to someone else esp bc they wanted to
charge me more money for all this but my engine was out of the car at this point.
eventually, maybe 4-5 weeks later they finished. i test drove it and it was shaking
pretty bad at idle. i asked them to look it over and they said it absolutely had nothing to
do with them. i thought maybe the timing was off bc it didnt shake before they worked
on it. they said for $100 they could diagnosis for me. now skeptical bc ive never been
charged for something like that after paying for work done before, i told them that was
a bit high, they lowered it to 45. told me my car was shaking bc of water in the gas
tank, need new spark plugs/wires and another oil change (they just did one but i guess
i need another?) and said they recieved the knock sensor code and showed me it. now
my spark plugs are all new. wires are not cut or burnt. ive never had a knock sensor
code before and the fact that they said that was the ONLY code i kinda had a feeling i
should take it to someone else for a second opinion. not to mention the water in my
gas tank bit. plus i had my codes checked directly before all this and it didnt show up.
but its there, on the scanner, maybe im wrong? i want to be certain its not the timing
belt. these guys were a little iffy after dealing with them so long i had a feeling..took it
to my regular shop. NOW..timing belt is off by 2 teeth. timing belt cover was missing
two bolts or screws/not installed correctly and it was warped bc of that and rubbing
against the belt itself. the knock sensor was UNPLUGGED..reason for code and the
battery was not hooked up properly. shop said it was unlikely but it could have fallen
over. i told these guys all of this, they said they were upset i took it to another shop,
and said they would correct it but that i need new spark plugs and another oil change. i
told them i didnt want all of that. just the correction (id like to go somewhere else-if i
dont just do it myself) they insist i need it and they want to do it (not for free of course.
but i just dont really trust them) they said they will redo the belt but they want money
directly afterwards....now id like to take it back to the shop before i give them anymore
money. total bill is about 900 and they have recieved 500 previously. i just dont want to
pay them off or give them anymore money until i know for sure the job is done because
i dont feel as if they will fix it if they have what they want. any advice? i paid 125 for
the shop to tell me the timing was off and i know for a fact im not giving these guys the
45 for that diagnosis. should i take the 125 off of what i owe them? i really hate to have
to go to court but i dont want the short end of the stick.