CMHC is the least of your worries as the lender submits the deal to them for approval. Normally the lenders or banks have tighter guidelines than CMHC so the lender can be assured of CMHC insurance when they submit the deal.

You do not have to. The insurance should only be on the top-up amount of the loan. Be careful though as CMHC may not agree on the valuation of the property if it is a top-up insurance and you may not get the property value you need to renew or refinance a mortgage.