1.If by "anti-freeze deposit" you mean the coolant reservoir which receives the overflow from the radiator, then a gash in it that does not cause you to loose coolant will not affect car temperature or performance. But a crack will generally get worse and overtime will probably affect an area of the reservoir that will cause coolant to leak out.
Solution - go to a salvage yard and get another one to replace it.
2.Text From GM TSB #01-08-44-005A - (Oct 8, 2004)
ˇ E20 or CHECK CD : Cause(*): The CD player cannot focus on the disc or there is an internal focus problem. Correction(*): Try cleaning the disc or using a new disc. Make sure the disc is loaded correctly. The road could be too rough or there is too much humidity. If the problem is not corrected, service the unit at an ACDelcoŽ Service Center.
Most of the error codes have that last part about sending it to the service center (I wouldn't expect an auto mechanic to fix my stereo - )
3. The check engine light can be about very many different things concerning your engine. Some are really unnecessary - IMHO [in my humble opinion]. You will need someone to hookup an OBCII (on board diagnostics) code reader to your OBCII connection and "read" the code for you and clear the code which will turn off the CEL. AUTOZONE does this for free if I understand correctly... Here is a sample of the SOME of the possible codes for several makes of vehicles:
You could also purchase the reader for less then $80USD and do it yourself.