What exactly do you want to do? Just flight instruction? You can do that with a PPL in Europe - if you have 200 hours and 150 PIC. You will need to have passed the CPL or ATPL exams, but you don't need to have a CPL. Of course, since you need 200 hours anyway, then to get an IR and/or CPL would only cost you the extra instruction. You'd be daft to just keep hour building.
You can be a LAPL instructor without the CPL exams.
The exams: hardly anyone take the CPL exams, because most people want to go on to fly for airlines - they just go straight to the ATPL exams. Most of the questions are very similar of not identical so you should go for the ATPLs. In fact, since there are more question banks and support for the ATPLs, ironically they are actually easier!
Once you have the exams they are valid for 3 years for CPL/IR, and indefinitely as long as you renew your IR every 7 years. They have no expiry date in respect of an FI course, you just have to do it once.