Well, let's look at some numbers. Republicans held the Senate in '84, losing two seats to go from 55-45 to 53-47. Since this was before the current "filibuster everything" era, there isn't really an incentive to drive for 60 seats (and the math doesn't support that, anyway).
So the only real potential advantage to Reagan in nationalizing the '84 election would be if he could help the GOP retake the House of Representatives.
This is, to put it mildly, an enormous alt-challenge. To win the House in '84, Republicans would have had to flip 51 seats; IOTL, they won 16. As I look at the competitive races, I don't see another 35 Republican challengers who could have plausibly won regardless of how much Reagan nationalized the election. Here's how it breaks down:
A) Three elections were 50/50: ID-2, in which Richard Stallings narrowly defeated George Hansen (R); IN-8, a contested race in which incumbent Frank McCloskey (D) defeated Dick McIntyre by at most a handful of votes after a recount (which I discuss in Dirty Laundry); and PA-7 in which incumbent Robert W. Edgar (D) narrowly defeated Curt Weldon (R), 50.1-49.9. Obviously, those three races could flip Republican.
B) Three more races were 51-49 in favor of the incumbent Democratic congressman: PA-8 (Kostmayer def. Christian), NC-5 (Neal def. Epperson) and NC-8 (Hefner def. Blake). Call those three flips as well.
C) Now Reagan needs 29 more seats. Unfortunately for him, there were only 22 other House races in '84 in which a Democrat won by "only" 10 or fewer points. Here's every one of them, with asterisks indicating incumbents:
AR-2: Tommy F. Robinson (D*) 47.1 - Judy Petty (R) 41.5
CA-30: Matthew G. Martinez (D*) 51.8 - Richard Gomez (R) 43.4
CO-2: Tim Wirth (D*) 53.2 - Mike Norton (R) 45.5
CT-2: Sam Gejdenson (D*) 54.5 - Roberta F. Koontz (R) 45.2
CT-3: Bruce A. Morrison (D*) 52.6 - Lawrence J. DeNardis (R) 47.2
IL-19: Terry L. Bruce (D) 52.3 - Dan Crane (R*) 47.7
IL-22: Kenneth J. Gray (D) 50.3 - Randy Pratchett (R) 49.7 (Paul Simon's seat)
IN-2: Philip R. Sharp (D*) 53.4 - Ken Mackenzie (R) 46.3
MA-5: Chester G. Atkins (D) 53.4 - Greg Hyatt (R) 46.6
MI-3: Howard Wolpe (D*) 52.9 - Jackie McGregor (R) 47.1
MI-6: Milton Robert Carr (D*) 52.4 - Tom Ritter (R) 46.7
MO-2: Robert A. Young (D*) 51.8 - John Buechner (R) 47.5
MO-9: Harold Volkmer (D*) 52.9 - Carrie Francke (R) 47.1
NJ-3: James Howard (D*) 53.3 - Brian Kennedy (R) 45.8
NY-2: Thomas J. Downey (D*) 54.7 - Paul Aniboli (R) 45.3
NY-3: Robert J. Mrazek (D*) 51.0 - Robert P. Quinn (R) 47.9
OK-1: James Robert Jones (D*) 52.2 - Frank Keating (R) 47.3
OR-1: Les AuCoin (D*) 53.1 - Bill Moshofsky (R) 46.9
TN-3: Marilyn Lloyd (D*) 52.4 - John Davis (R) 47.6
VA-6: Jim Olin (D*) 53.5 - Ray L. Garland (R) 46.5
VA-9: Rick Boucher (D*) 52.0 - C. Jefferson Stafford (R) 48.0
WV-1: Allan Mollohan (D*) 54.4 - Jim Altmeyer (R) 45.6
Worse, of these potentially close races, a number feature liberal Republicans running in Democratic-leaning districts where a nationalization of the election would probably hurt, rather than help the candidate: CA-30, CO-2, IL-19, IL-22, MA-5, and NY-2. Others feature very strong Democratic incumbents with solid leads, like OR-1 and WV-1.
Obviously, there are some seats here that are ripe for the picking -- both Missouri districts, Arkansas's second district, and future governor (and criminal) Frank Keating in Oklahoma's first district. But beyond that, these are very tough nuts to crack.
So even a good full-court national press by Reagan probably could only net the Republicans an additional ten seats, which is just not enough to make a difference. On the flip side, here are the states Mondale lost OTL by 10 points or fewer: Massachusetts (-3), Rhode Island (-3), Maryland (-5.5), Pennsylvania (-7), Iowa (-7), New York (-8), and Wisconsin (-9). Iowa and Pennsylvania are more tossup states, but the other five are deep blue states in '84 that could have easily flipped Democratic if the race had been nationalized on Republican issues. California (-16) is a longer shot, obviously, but there's fairly good evidence that late voting broke Republican (because the election was called at 5 pm Pacific time) and California has the strongest divide between personal approval for Reagan and disapproval for national Republican policies.
Bottom line: I think Reagan was in a stronger position in '85 having won a historic landslide but facing a strongly Democratic House of Representatives than he would have been had he defeated Mondale, say, 395-143, but picked up another 10 seats in the House.