It's impossible for all of your compensation to be deferred to the 457(b). Since some will go to income tax, Social Security and Medicare withholding, only what remains after that withholding will be deferred to the 457(b). This means that you'll have at least some amount of compensation remaining from which to make a Roth IRA contribution, the amount in box 1 of your W-2. The maximum amount of your contribution will be the lesser of your remaining compensation and the maximum amount that you are eligible to contribute to a Roth IRA based on your age, federal tax filing status and your modified AGI for the purpose. See IRS Pub 590-A:
There is no limit on the amount that you are able to convert to a Roth IRA from your rollover IRA and such a Roth conversion has no effect on how much of a regular contribution you can make to a Roth IRA.