In all honesty I can say that I have never seen anyone prevail by using an MUTCD argument. While the MUTCD does purport to be the standard for traffic control devices, signage, markings... etc, and while many people misinterpret its placement on the hierarchy scale, it often ensures that it fulfills the statutory requirement for signs and signals and often times it may enhance on those requirements. But for all intents and purposes the statutory requirement is what will hold up in court. In fact, the MUTCD has some statements that make this basic principle quite clear:
This Manual describes the application of traffic control devices, but shall not be a legal requirement for their installation.
So going back to the code section itself, which you can read it in its entirety HERE, the signage/marking requirement (underlined) comes under subsection (b), and the prohibition (bolded) is also under subsection (b):
21655.5.
(b) The Department of Transportation and local authorities, with respect to highways under their respective jurisdictions, shall place and maintain, or cause to be placed and maintained, signs and other official traffic control devices to designate the exclusive or preferential lanes, to advise motorists of the applicable vehicle occupancy levels, and, except where ramp metering and bypass lanes are regulated with the activation of traffic signals, to advise motorists of the hours of high-occupancy vehicle usage. No person shall drive a vehicle upon those lanes except in conformity with the instructions imparted by the official traffic control devices. A motorcycle, a mass transit vehicle, or a paratransit vehicle that is clearly and identifiably marked on all sides of the vehicle with the name of the paratransit provider may be operated upon those exclusive or preferential use lanes unless specifically prohibited by a traffic control device.
And as you can see, it requires "signs and other traffic control devices" and from the Google Maps link for the on-ramp, it appears that there are at minimum 3 signs and several pavement markings and so the "notice" requirement is satisfied.
It appears that the OP was trying too hard to come up with some legit argument... Either that or I am confused about something. In one sentence he says he didn't want to slow down for fear he might cause an accident, all while he is trying to convince us that there was a traffic back up and he couldn't use the non-HOV lane because it was backed up, but that he also turned simultaneously with a driver who was turning right on the on-ramp... None of it is going to help justify his actions, in my opinion. I really don't see this being any different from the other two or three HOV lane citations we've recently discussed. Driver tries to circumvent a traffic back up, gets caught and is now back pedaling.
Either way, it does not appear to be as bad as he wants to make it out and it certainly isn't likely to be as nightmare-ish as your scenario, In fact, here is a link showing that the meter is on, and yet if I traverse backwards from the meter, as I did in this link, you'll see that the back up isn't that there (both lanes are completely empty), it is not that bad at all. I cannot claim that this presents the same conditions at the time he used the on-ramp, (of course this is only one moment it time when these images were taken) but if the meter is on, that is likely as bad as it will get on a normal day.
My other guess is if the non HOV lane was really backed up as the OP describes, he would have at least mentioned it to the citing officer. In this case, he simply "did not rebuttal to the CHP officer at all and agreed to everything he said since either way [he] would be given a citation for being in a carpool lane".
With that being said, and although it appears that you did a bit of research on this one, and while I hate to disappoint you or make it seem like your work was in vain but you'll have to note that a driver turning left from the OPs direction onto the on-ramp is facing a left turn arrow. And when it shows green in his direction, opposing traffic has a red signal. And so while my initial assessment was that he also violated the right of way, I was clearly way off in that simply because he would have the right of way when he is able to get across the intersection and if as he claims, there was another car turning in the right lane, presumably he would have told the officer about this as well, and/or the officer would have seen that alleged violation and either cited it as well or cited it instead since it would have affected the OPs movement and forced him into the HOV lane!