An Arrow too far?

‘Challenge Ratings’ are pretty lame. Would 17th level Estaban have lasted more than 12 seconds if he stood against the Party alone?
The way I see it is that a 'Challenge rating' equal to the Party is only a quarter as powerful.
Just out of curiosity though, would one of you ‘Techies’ work out what the ‘Challenge Rating’ was for the final battle?

The Party:

1) Eolar: 17th level Cleric/Ranger
2) Thesis: 17th level Fighter/Barbarian
3) Bodush: 16th level Sorcerer
4) Durin: 14th level Barbarian
5) Mara'ch: 13th level Rogue/Sorcerer thingy

Party assets:

1) Ragnar: 13th level Fighter/werebear (Don’t say he doesn’t count just because he’s a sidekick)
2) Roly: 5th level Cleric (Would have healed ‘certain’ Party members)
3) Lizard-folk slaves. x 2 (CR:2)

Neutrals:

1) Meldo
2) Children x 7
3) Siren children x 3
4) Sailors x 3
5) Hamster-men x 2

Opposition:

1) Estaban: 17th level Bard (With only one offensive spell that he could only use once)
2) Dorgon: 12th level Fighter. (Meant to have targeted Mara'ch)
3) Valdis: 13th level Fighter/Barbarian (Silver-sheened Halfling of death!)
4) Mendoza: 12th level Fighter (Hit and run specialist)
5) Hawkmoon: 14th level Fighter/Wizard (Traitorous turncoat)
6) Mendez: 17th level Fighter/Barbarian/Ranger (Geased to attack any that attacked family)
7) Benidict: 7th level Rogue (Law hating backstaber)
8) Eris: 3rd level Wizard (Sweet 18 year old little sister)
9) Brothers x 11: 4th level Fighters (Completely safe on their 10' wall...)
10) Thugs x 12: 1st level Fighters (Completely worthless)
11) Servants x 4 1st level commoners (Even more completely worthless)

As for the encounter setting...
Estaban had planned against you but the courtyard itself was only a minor obstacle and the tactics were pretty straight forward.