Volvo Forums - Volvo Enthusiasts Forum - 99 volvo o2 sensor issue turns into bent valve

I have a 99 s80 T6 with only 61,000 miles on it. My check engine light came on so I took it to my mechanic. I've been dealing with this mechanic for a while and he has always helped me out. He told me that it was an o2 sensor problem. He went ahead and changed the o2 sensor and the car seemed fine the first day I got it back. The next morning I started the car up and it started to smell like rotten eggs. I then noticed the car was idling very rough and the check engine light came back on. I took it to my mechanic and he told me that he thought the cats were clogged up and that was what was causing the smell and the idle issue. The car drove very ruff and would not go over 30 mph. To replace both cats was around $600 so I was weary on doing it. I asked the mechanic if he could guarantee that it would fix the problem and he said "yes I'm 100% positive it will. After replacing the cats the car did the same exact thing. I took the old cats to him and he said the cat on the right side was clogged because it was blacker than the other one on the inside of the cat. After this he checked the plugs to see if they were fouling out and said 3 plugs were fouled out on the right side. He changed the plugs and it didn't fix anything. The car still had a check engine light on and this time it was more than one code. This time it had another o2 sensor code and some cylinder misfire codes and a camshaft position sensor switch. He then said the timing was off so he then changed the timing belt. After he did that the timing was off. He then did some research on the problem and said we needed to change the CVVT. I guess this is a solenoid on the camshaft? Anyways it was around a $150 and I went ahead and told him to do it. This still didn't fix anything. At this point I was very uncertain he could fix my car but figured he could do this last thing. After he did those things, he then got a new coil for the side that the plugs were filed out. This made the car run a lot smoother and the only thing that was wrong was code 14, timing off. Then he said the timing belt needed to be moved back a couple notches. While he was doing this the car made a loud noise (banging noise) and then he called me saying the car bent a valve or multiple valves. Now he needs to get head gaskets and replace the valves. After calling Volvo he found out that the timing belt needed a special tool in order to be timed correctly.

My question is why did the car run so bad from changing an o2 sensor? I was told that it needed to be a Bosch replacement or it wouldn't work and it was! Also was replacing the cat, CVVT, and timing belt necessary at all to fix the problem my car was having? Does this car need a special tool to set the timing? Could improper timing cause a car to bend a valve? It sounds like to me the only problems were the 02 sensor, fouled plugs and the bad coil. I think that would of fixed the car. The code 14 came up after he put the new timing belt on. I really need some answers from someone else than my mechanic because I can't spend anymore money without finding out what I need to do to fix this car. Any help is appreciated.