With the utilt, are you saying the hitboxes cause it to fail because of shield poke or due to not being able to stack shield stun to an already stunned shield? If it's the former case I'll take your word, but for the latter I think there are still enough stun frames to allow for true pressure.
Regarding bair:
IC's bair autocancels after 19 and, with a perfect fastfall, the shorthop is 21. With landing lag, this bring the total frame count of the iteration to 25. A bair will give 14 frames of shield stun. Now, since ICs can start an attack again halfway through its iteration via desync, the formula to determine (theoretically) whether it could work would be
Shield stun must be >= (1/2)iteration {+ human error but disregard this for the sake of theory}
14 > 12.5
In reality, this is probably irrelevant because you'd need perfect desynced shff'd bairs that incorporate the 6 frame Nana delay, but yeah. Unless I missed something there I think that the bair is theoretically viable? I didn't check any other moves beyond utilt and bair.
Edit: okay, scotu-style. Green represents shield-stunned, red is non shield-stunned. I need to go back and add in notes for the actual frames you'd have to input Nana's commands in for it to be frame-perfect, but I've given myself a bit of a headache and can't be arsed to right now.
Popo
1 Start jump
2
3
4 Airborne, start bair
5
6
7
8
9
10
11 Bair hits
12
13
14
15
16
17
18
19
20
21
22
23
24
25 Land, Landlag
26 Landlag
27 Landlag
28 Landlag
29 Start jump
30
31
32 Airborne, start bair
33
34
35
36
37
38
39 Bair hits
40
41
42
43
44
45
46
47
48
49
50
51
52
53 Land, Landlag
54 Landlag
55 Landlag
56 Landlag
Nana
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15 Start jump
16
17
18 Airborne, start bair
19
20
21
22
23
24
25 Bair hits
26
27
28
29
30
31
32
33
34
35
36
37
38
39 Land, Landlag
40 Landlag
41 Landlag
42 Landlag
43 Start jump
44
45
46
47
48
49
50
This represents frame-perfect iterations, and, if I've not screwed up anywhere along the way (someone please check for that), this is exactly perfect? Popo stuns from 11-24, Nana from 25-38, Popo from 39-52, etc.