1) You can file your 2018 tax return without waiting for your 2017 tax return to be completed, but ask your CPA if there is anything in the 2017 tax return that might affect your 2018 tax return, such as a carryover. If there is, then you should wait to file 2018 until 2017 has been completed.
2) It depends on the CPA's fee schedule or the agreement that you have with him. There is nothing in the tax law or IRS regulations that defines how long he can take to complete an already-late tax return, or whether you have to pay him for the time he spends on a tax return that you do not file.