Hi RitaBB,
Sorry that you have lost your husband.
Re: "I am in the process of shutting down my late husbands apple account".
and
"I understand the device will be tied into what will be a no existent account".
Wondering if this Apple Support article (link below) may help:
How to request access to a deceased family member's Apple account - Apple Support
Eg: If you gain access, you may want to make account changes.
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re: "I have forgotten my admin password for a late 2012 Yosemite".
How to access older Mac without Admin password,
(eg: using Apple ID from above process):
Change or reset the password of a macOS user account in macOS Mojave or earlier – Apple Support
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If I want to sell on or re use with OS X installed how can I do this ?
To properly prepare a Mac for a new or different) owner:
See: What to do before you sell, give away, or trade in your Mac - Apple Support
All the best :-)