The deadline to roll over the offset distribution is the due date of the tax return including extensions.
The offset distribution can be rolled directly to a Roth IRA, which would be taxable, or it can be non-taxably rolled over to a traditional IRA and then taxably converted to Roth. As long as you end up with $0 in traditional IRAs at year-end after having converted everything to Roth, which appears to be what you are suggesting, mixing the pre-tax funds from an offset rollover to a traditional IRA and nondeductible traditional IRA contributions is not a problem.
