Good day! Thank for posting in the community.
We've received your query, and you can expect to receive response from one of our community moderators accordingly. I am sorry to see that you cannot add external users to a shared channel in Teams.
Per your update, i would assume you have confirm these settings already
= B2B Direct Connect - Ensure that B2B Direct Connect settings are properly configured, ensure that B2B Direct Connect settings are properly configured Re: Shared channel not adding external users (One way issue) - Microsoft Community Hub
= Cross-Tenant Access Settings- Check the inbound and outbound settings for cross-tenant access in Microsoft Entra ID, i guess you already reviewed this part right per your update Shared channels errors in Microsoft Teams - Microsoft Teams | Microsoft Learn
= Verify Teams Channel Policies- Make sure the policy for inviting external users to shared channels is turned on
In the meantime, please note that shared channels don’t work with consumer accounts (Gmail/Hotmail). Only business accounts since they rely on B2B direct connect not B2C.
Secondly the admin in the other external organization also has to configure their cross-tenant settings on their end to allow inbound and outbound before you can invite or share the channel to their users. https://learn.microsoft.com/en-us/entra/external-id/cross-tenant-access-settings-b2b-direct-connect
If all the above settings have been well configured, update me one more time so that we can move to the next step, look forward to hearing from you, thanks.