Can people who are not a 74D wear the CBRN patch?

No, and you'll save some ridicule not wearing one.
The answer is in the message when it was originally approved.
ALARACT 195/2008: "THE SUBDUED CBRN INSIGNIA IS WORN WITH UTILITY UNIFORMS BY CMF 74/BRANCH 74 PERSONNEL ENGAGED IN CBRN OPERATIONS DOMESTIC AND IN THE FIELD WHEN AUTHORIZED BY THE LOCAL COMMANDER."
A CBRN operations domestic example would be a 74D WMD CST member. In the field example would be in an operations center to rapidly identify the CBRN personnel (as if most folks wouldn't know who was who without a brassard).
The CBRN brassard is a joke among many CBRN NCOs, who will give the side eye to a 74D wearing one, much less anyone else.

No.

Commander discretion. I certainly never ran into any company CBRN NCO who was allowed the patch unless they were that MOS, even as a secondary MOS.

Not busting on you, But you should check with your Schools NCO about the Two week wonder corse's that are tought evry now and then. Or the Chem and bio Casualty course. I think it is still tought at FORT MEAD.

I'd check out this discussion too:
https://www.rallypoint.com/answers/what-are-the-regulations-on-the-wearing-of-the-cbrn-patch

I can't find anything stating that you can't wear it in regards to your particular case. That being said, if you are in fact authorized to wear it due to being the CBRN NCOIC, then per AR 670-1 and DA PAM 670-1:

Brassards are worn as ID to designate personnel who are required to perform a special task or to deal
with the public. Brassards are authorized for wear only while actively engaged in the duty associated with the brassard and identification of personnel is required, such as field operations and event response. Brassards are not intended for wear while performing daily or routine job related activities. For example, a member of an Explosive Ordnance Disposal (EOD) Team would wear the EOD brassard while actively conducting disposal operations in an environment where non-EOD personnel are present and identification of EOD personnel is necessary; the brassard is not worn while conducting staff
activities, routine maintenance and preparations, or while in an on-call or stand-by status