Firstly how can you instruct PPL students and not do any navigation.
Secondly you can only instruct IMC students once the restriction has been removed so you will be instructing VFR for a while anyway.
Third, what school will pay for a ME rating anyway that surely (dont call me Shirley)is down to you.
About the bond-I dont think they can legally do that as they are not paying for anything.(or are they)Best check that out.If you do sign and it is not a legal contract then you can obviously go.