What to expect after replacing a idle speed or air control valve and getting a throttle body cleaning?

I recently had 2 diagnostics done and both said I needed to replace the idle air control valve but then one also said I needed a throttle body cleaning so I had my friends dad replace the part and do the cleaning after he did that the car still drove a lil funny like before it would shut off at red lights or make the sound like it was going to cut off then I would have to step all the way on the gas to the floor for it to make any movement and so right after he cleaned the throttle it shut off on me and he said to keep the car running cause it had to get used to the chemical and so I did and then I went driving it around and it stopped making the noise but it still is a drag when I switch from reverse to drive or vice versa when is it going to start acting normal again and I have a 1993 mercury tracer kind of like a ford escort and also if a back tail light goes out would that cause the check engine light to come on or no? And also why does it cost to have a diagnostic done on an older car like it's almost a 100$ everytime I want to get one everywhere I go it's only free for cars 1997 and up I like my old car wish it wasn't harder to figure out the problems tho.

If idle speed control valve was replaced. Then we don't need any cleaning chemicals.
Your symptoms sound like air vacuum leak. Did we replace gaskets? Check vacuum lines too. Try water with washing up liquis sprayed over induction joints.hoses.
Or starter fluid. If engine for example speeds up or revs change markedly when applying fluid? We have found our leak source.
Other possibility is the auto fuel enrichment device isn't working. This is a cars system for increasing fuel on idle speed to stop stalling and cutting out when putting an auto on gear due to load on engine.
Tail light not related at all.
Older cars have less competition for diagnostic checks and offer more scope for fees due to not everybody having scanners that can read etc.
May be best to think about getting own.