I think the 1st question to consider is: What moves should even keep you in shield stun the entire time when they are fresh? As far as Melee goes, not many multihit moves actually do this at full strength, and it's generally only the extremely rapid ones and/or slightly more damaging ones such as the 3+ damage per hit drills.
Also no, most staled multihit attacks generally shouldn't lock you in shield, but again it really depends on the attack itself, and the stale codes being used. With the stale reductions only at like 1/4th, there really won't be much of a significant reduction in damage for multihit attacks as they do very little damage per hit to begin with, and in turn it won't have much of an impact on shieldstun in most cases.
I remember you mentioning before about CF's n-air not keeping them in shieldstun between the 1st to 2nd kicks. His kicks actually didn't even come close to doing this before. I'll give some examples of moves down below.
A lot of the hitbox data for attacks stayed the same between the 2 games, so if there isn't very detailed frame data for a move's hitboxes in Brawl, there's a good chance the hitbox timings are the same as they were in Melee anyway. When I did Luigi's data I ended up just copying the listing for him from Melee and not needing to change much for the hitbox info. It was mostly the other properties of the moves like lag and such that were different.
Damage changed quite a bit in some cases too which is important. Peach's D-Smash is an obvious example of this. In Melee at 14% a hit and only 4 frames apart it locked down shields with plenty of stun to spare.
For the 'Shieldlock' part, positive numbers shield lock, while 0 and negatives do not:
+X: They are locked in shieldstun for an additional X number of frames beyond when the next hit connects (shield locks).
+1: There's just enough stun so that they cannot release shield before the next hit connects (shield locks).
0: It can be released at the same time the next hit connects, so if they let go of shield between hits they get hit (no shield lock).
-X: They are able to drop it X number of frames before the next hit connects (no shield lock).
Also, Electric-Type attacks have 1.5x hitlag for the opponent, but only on HIT. On block they have normal hitlag.
Bowser
D-Air
Hit: 14-15, 17-18, 20-21, 23-24, 26-27, 29-30, 32-33, 35-36, 38-39
Damage: 3 (Hits 1-9)
Hitlag: 4 (Hits 1-9)
Shieldstun: 3 (Hits 1-9)
Shieldlock: +1 (Hit 1 -> Hit 2); etc
Captain Falcon
N-Air
Hit: 7-12, 20-29
Damage: 6 (Kick 1); 7 (Kick 2)
Hitlag: 5 (Kick 1); 5 (Kick 2)
Shieldstun: 4 (Kick 1); 5 (Kick 2)
Shieldlock: -8 (Kick 1 -> Kick 2)
Fox
D-Air (sweetspot)
Hit: 5-6, 8-9, 11-12, 14-15, 17-18, 20-21, 23-24
Damage: 3 (Kicks 1-7)
Hitlag: 4 (Kicks 1-7)
Shieldstun: 3 (Kicks 1-7)
Shieldlock: +1 (Kick 1 -> Kick 2); etc
There's also a weaker hitbox on the d-air at the end of his feet that only does 2 damage:
D-Air (sourspot)
Hit: [Same]
Damage: 2 (Kicks 1-7)
Hitlag: 3 (Kicks 1-7)
Shieldstun: 2 (Kicks 1-7)
Shieldlock: 0 (Kick 1 -> Kick 2); etc
Rapid-A Kicks
Hits: 3-4, 10-11, 17-18, 24-25, 31-32
Damage: 1 (Kicks 1-5)
Hitlag: 3 (Kicks 1-5)
Shieldstun: 2 (Kicks 1-5)
Shieldlock: -4 (Kick 1 -> Kick 2); etc
Game & Watch
B-Air
Hit: 10-21; Hitbox refreshes and can hit again every 3 frames, so 10, 13, 16, 19
Damage: 5 (Hits 1-4)
Hitlag: 4 (Hits 1-4)
Shieldstun: 4 (Hits 1-4)
Shieldlock: +2 (Hit 1 -> Hit 2); etc
Jigglypuff
D-Air
Hit: 5-6, 8-9, 11-12, 14-15, 17-18, 20-21, 23-24, 26-27
Damage: 2 (Kicks 1-8)
Hitlag: 3 (Kicks 1-8)
Shieldstun: 2 (Kicks 1-8)
Shieldlock: 0 (Kick 1 -> Kick 2); etc
Kirby
D-Air
Hit: 18-19, 21-22, 24-25, 27-28, 30-31, 33-34
Damage: 3 (Kicks 1-6)
Hitlag: 4 (Kicks 1-6)
Shieldstun: 3 (Kicks 1-6)
Shieldlock: +1 (Kick 1 -> Kick 2); etc
F-Air
Hit: 10-11, 17-18, 25-26
Damage: 5 (Kicks 1-2); 8 (Kick 3)
Hitlag: 4 (Kicks 1-2); 5 (Kick 3)
Shieldstun: 4 (Kicks 1-2); 5 (Kick 3)
Shieldlock: -2 (Kick 1 -> Kick 2); -3 (Kick 2 -> Kick 3)
Link
B-Air
Hit: 6-9, 18-23
Damage: 7 (Kicks 1-2)
Hitlag: 5 (Kicks 1-2)
Shieldstun: 5 (Kicks 1-2)
Shieldlock: -6 (Kick 1 -> Kick 2)
Mario
D-Air
Hit: 10-11, 13-14, 16-17, 19-20, 22-23, 25-26
Damage: 2 (Kicks 1-6)
Hitlag: 3 (Kicks 1-6)
Shieldstun: 2 (Kicks 1-6)
Shieldlock: 0 (Kick 1 -> Kick 2); etc
Marth
N-Air
Hit: 6-7, 15-21
Damage: 4 (Hit 1); 10 (Hit 2)
Hitlag: 4 (Hit 1); 6 (Hit 2)
Shieldstun: 3 (Hit 1); 6 (Hit 2)
Shieldlock: -5 (Hit 1 -> Hit 2)
Ness
F-Air (Electric-Type)
Hit: 8-10, 11-13, 14-16, 17-19, 20-21
Damage: 3 (Hits 1-4); 5 (Hit 5)
Hitlag: 4 (Hits 1-4); 4 (Hit 5)
Shieldstun: 3 (Hits 1-4); 4 (Hit 5)
Shieldlock: +1 (Hit 1 -> Hit 2); etc
Peach
D-Air
Hit: 12-13, 18-19, 24-25, 30-31
Damage: 3 (Kicks 1-4)
Hitlag: 4 (Kicks 1-4)
Shieldstun: 3 (Kicks 1-4)
Shieldlock: -2 (Kick 1 -> Kick 2); etc
D-Smash
Hit: 5-6, 9-10, 13-14, 17-18, 21-22
Damage: 14 (Hits 1-5)
Hitlag: 7 (Hits 1-5)
Shieldstun: 8 (Hits 1-5)
Shieldlock: +5 (Hit 1 -> Hit 2); etc
Pikachu
D-Smash (Electric-Type)
Hit: 7-8, 10-11, 13-14, 16-17, 19-20, 22-23, 25
Damage: 2 (Hits 1-6); 3 (Hit 7)
Hitlag: 3 (Hits 1-6); 4 (Hit 7)
Shieldstun: 2 (Hits 1-6); 3 (Hit 7)
Shieldlock: 0 (Hit 1 -> Hit 2); etc
Samus
F-Air
Hit: 4-5, 11-12, 18-19, 25-26, 30-31
Damage: 5 (Hits 1-5)
Hitlag: 4 (Hits 1-5)
Shieldstun: 4 (Hits 1-5)
Shieldlock: -2 (Hit 1 -> Hit 2); ... 0 (Hit 4 -> Hit 5)
Yoshi
D-Air
Hit: 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44
Damage: 4 (Hits 1-14)
Hitlag: 4 (Hits 1-14)
Shieldstun: 3 (Hits 1-14)
Shieldlock: +2 (Hit 1 -> Hit 2); etc
Zelda
U-Smash (Electric-Type)
Hit: (1st wave) 5, 7, 9, 11, 13, 15, (2nd wave) 24, 26, 28, 30, 32, 34
Damage: 1 (Hits 1-11); 5 (Hit 12)
Hitlag: 3 (Hits 1-11); 4 (Hit 12)
Shieldstun: 2 (Hits 1-11); 4 (Hit 12)
Shieldlock: +1 (Hit 1 -> Hit 2); ... -6 (Hit 6 -> Hit 7); +1 (Hit 7 -> Hit 8); etc
F-Smash (Electric-Type)
Hit: 16, 18, 20, 22, 24
Damage: 1 (Hits 1-4); 14 (Hit 5)
Hitlag: 3 (Hits 1-4); 7 (Hit 5)
Shieldstun: 2 (Hits 1-4); 8 (Hit 5)
Shieldlock: +1 (Hit 1 -> Hit 2); etc
N-Air
Hit: 6-7, 10-11, 14-15, 18-19, 22-23, 26-27
Damage: 2 (Hits 1-5); 5 (Hit 6)
Hitlag: 3 (Hits-5); 4 (Hit 6)
Shieldstun: 2 (Hits 1-5); 4 (Hit 6)
Shieldlock: -1 (Hit 1 -> Hit 2); etc
Nayru's Love
Hit: 12, 15, 18, 21, 24, 27
Damage: 1 (Hits 1-5); 5 (Hit 6)
Hitlag: 3 (Hits 1-5); 4 (Hit 6)
Shieldstun: 2 (Hits 1-5); 4 (Hit 6)
Shieldlock: 0 (Hit 1 -> Hit 2); etc