I regret throwing that Sea Foam can away now, but doesn't the can say it will not harm sensors ect.?

 

Sea Foam or Mopar Combustion Chamber Cleaner should have no effect on the sensors. I'd be very curious what 3 sensors the shop felt needed to be replaced, and how they arrived at that conclusion. Is the CEL lit? Did you do an oil change immediately after (as recommended)?

 

It sounds like the guy is trying to give you the shaft, if he seriously things that there are three O2 sensors on the 1st gen neon then he doesn't need to be doing any mechanical work period, the O2 sensors are easy to change and shouldn't cost that much, and that egg smell could still have something to do with the cat even if he checked it out I wouldnt trust this guy to much Id also like to know the garage came to there conclusions as well

 

Ok, so tell me if this is something caused by a shot O2 sensor: Going down the road, depress the clutch, it idles at about 1800-2000rpm, difficulty starting when it is warm, when it is stopped, it idles a bit rough. When I start it, it bogs down to about 500rpm and idles extremely rough and I have a bit of backfireing going on here and there

 

A shot 02 sensor wouldn't cause your car to do that, the upstream sensor just adjusts your fuel ratios. and the downstream just monitors your cat if anything the upstream would cause you to run lean. it could be that you might just need a tune up, especially since you said it goes down to 500 rpm which could be from sparks being fowld, something with the coil pack, possible fuel pump problem, water in the tank, A vacum problem or even dirty injectors. these are just some possible things that could be causing some problems I hope some of that info was helpful, Best of luck