With viewers clearly suffering JFK overload, ABC’s “JFK: Reckless Youth” fell to what could be a historic low opening for a network miniseries, according to Nielsen overnight ratings, as CBS dominated Sunday with its nostalgic “A Walton Thanksgiving Reunion.”Coupled with ABC’s weak finish Saturday and inflated CBS numbers thanks to a special two-hour “Dr. Quinn, Medicine Woman,” those Sunday results may be enough to push CBS into the lead in the November sweeps after trailing the Alphabet network throughout the first 2 1/2 weeks of the monthlong survey. The ABC cause won’t be helped if “JFK” lives down to its first two hours, which captured just an 8 rating/12 share in the Nielsen metered markets representing about half of the U.S. TV households, compared with a 17.7/26 for the “Waltons” telepic and 14.7/22 for NBC’s movie “A Family Torn Apart.” Part 2 of “JFK” airs tonight from 9-11 p.m. The two-parter followed a series of network and cable programs related to the Kennedy assassination, including a pair of two-hour specials on CBS last Wednesday and Friday. Both of those programs put up modest numbers as well, the latter opening the door for ABC. The Alphabet web equaled its best national Friday rating of the season and achieved its highest regular-series numbers in more than 2 1/2 years from 9-10 p.m. with “Step by Step” (14.5/25) and “Hangin’ With Mr. Cooper” (13.9 /24). The weekend results should also help CBS break a three-week ABC winning streak in the weekly national Nielsens, which are due out today. The two webs have split the opening weeks of the ’93-94 season with four victories apiece.