Here's a video from Scotty Kilmer about the P0300 code. He gives you a couple of things to look for.

He's got some useful videos and some not so useful. I enjoy watching him though.

Your car obviously has 6 coil packs and the first two mechanics are idiots.

I'm not sure why Toyota says its a bad injector on cylinder 6. (It could be--I just don't know how they came up with that diagnosis.) I'd be interested as to how he came to this conclusion since the P0302 code is for cylinder 2 and the P0303 is for cylinder 3.

I think cylinder 6 is in the front, so if you need to change that one out, its more accessible. Double check on which cylinder #6 is. I don't have my book with me.

There should be a bunch of on line tutorials about changing out injectors. See what youtube has. Most V6 Toyota videos on you tube are the same 1mz-fe engine that are in the siennas, camrys and some Lexus. There are even a few videos on DIY injector cleaning. You're dealing with removing a fuel rail and orings and leaks, so you have to be careful. I don't recall if you have to remove the plenum to get to the back injectors. Probably you do.

If it were me, I'd do the things the video linked above mentions, then I'd run a couple of cans of injector cleaner through the car to see if things don't improve. I'm not sure about the diagnosis given the codes for other cylinders. I'd call my service adviser at Toyota and ask him to explain to me how it is injector #6 that needs to be replaced under the circumstances.

 

Post this in the Gen3-4 Camry section. That forum gets a lot more traffic and the servicing of axles, trasmissions, engine is very similar between the V6 Camry and the Sienna. You are more likely to get a response there. I have started posting most of my questions there with much better results.