I think the easiest thing to do would be to create your mod folder and simple move all the regular campaign stuff over to it.
http://www.twcenter.net/wiki/-mod:my_mod
Beginners Guide to Modding
Then, go in and in only the descr_strat file remove reference to the senate, brutii, and scipii from the faction list.
Then, go down and cut and paste all the settlements from each of these factions into the appropriate area in the julii section.
Next, go up and simply ; out the whole sections for senate, brutii, and scipii. (I say do it this way so you can go back and replace them if you run into trouble.
Then go all the way to the bottom and remove the references to the factions you don't want from the faction relation areas.
That should be all it takes to get those three factions out and put all the Roman settlements into one faction. If you start it as a Provincial Campaign you wont have to worry about the senate or anything.
Also, you may be close. If the game is loading and simply saying you lost, then you probably did everything right except that you don't have a leader coded into the faction. Make sure you have at least one character with the leader tag in it.
Code:
character Cemendur, named character, leader, age 50, , x 105, y 71
Just had that happen last night, was editing the characters and forgot to put a leader in, and even though I had an heir, as soon as the game launched it said my faction was dead.
Good luck