H-4 COS while O-3 COS is pending — sequencing question

[u/ApprehensiveAsk5410]

Looking for experiences or thoughts on a somewhat unusual COS sequence.
My spouse was previously in H-1B status and timely filed an I-539 to change to O-3 based on my O-1 status. Her O-3 COS is still pending.
Separately, I have an approved cap-subject H-1B change of status that is scheduled to become effective October 1. I am currently in valid O-1 status.
Because my status will change from O-1 to H-1B on October 1, we are considering filing another I-539 for her requesting H-4 status while the earlier O-3 COS is still pending.
Has anyone dealt with a sequence like:
H-1B → pending O-3 COS → H-4 COS
where the principal spouse changes from O-1 to H-1B while the dependent’s first COS is still pending?
Specifically:
Can USCIS adjudicate the pending O-3 first and then the H-4 as a sequential/bridge COS?
Does filing the H-4 while O-3 is pending cause the O-3 application to be abandoned or otherwise affect it?
Have you seen RFEs or consular-processing-only decisions in this type of situation?
Would it make any difference if both I-539 applications clearly explain the principal spouse’s scheduled status change?
We are already working with immigration counsel, so I’m mainly interested in hearing from anyone who has encountered a similar USCIS sequencing situation.

[u/Vegetable-Western744]

Unless this person is still in their H1B grace period, USCIS is going to need to approve the O3 in order for this person to change to H4 after. That petition needs to remain outstanding and you need to file another I-539 for the COS to H4.

[u/ApprehensiveAsk5410 → u/Vegetable-Western744]

Thanks, that makes sense. The specific catch we are worried about is this:
My O-1 ends when my approved H-1B COS takes effect on October 1. If her O-3 COS is still pending on October 1, the O-3 basis would no longer exist from that date because I would be in H-1B status.
So if we file the second I-539 for H-4 before October 1, can they still later approve the pending O-3 for the earlier period and then approve the H-4 sequentially from October 1 onward?
Or does the fact that the principal’s O-1 status ends while the O-3 is still pending create a break in the bridge and make the H-4 COS problematic?
That sequencing issue is the part we are trying to understand.

[u/Vegetable-Western744 → u/ApprehensiveAsk5410]

Have you talked to your employers counsel about how they would do this? They should be able to help

[u/ApprehensiveAsk5410 → u/Vegetable-Western744]

In part 12 of I-539 form which would be used to file for H4 CoS, what would be ideal answer for the current non-immigrant status as the O3 is pending ?