Can Federal LEO w/20 years LEO service, age 45, step down to non-LEO federal job and retire at 50

The simple answer is absolutely. Here, for your information, is the reference.
In Chapter 46 of the CSRS FERS Handbook, https://www.opm.gov/retirement-center/publications-forms/csrsfers-handbook/, Section 46A3.3-1C (page 24) states “The employee must be separated from a position covered by retirement deductions. NOTE: An employee is not required to separate from a law enforcement officer or firefighter position to retire under the special group provision. Once an employee meets the minimum service requirement, he or she may exercise the right to retire under the special group provisions even if the employee is no longer covered by these provisions at the time of retirement.” This simply means you remain with the federal government and can retire when, in your example, you reach age 50 and will be eligible for all the benefits of the special provision group such as receiving the FERS Retiree Annuity Supplement and Cost of Living Adjustments.
Your second question “If so, what pay rate would I be paid at that lower grade.” The pay rate of the new position would dictate your new salary, however, if you meant how would your high-3 be calculated, the Office of Personnel Management always uses the highest 36 months of consecutive pay throughout your career, be it your last three years or sometime before that.