RE: Allow external users i.e. clinets to see the CRM case details
Hi partner,
For your first question--"The new case create form is not showing "entitlement" field "
(1)Enabled the Site Setting named CustomerSupport/CaseEntitlementEnabled to true

(2)In most cases where data doesn't appear on the portal has to do with table permissions, check if you have setup a table Permission for the entitlement entity, and it will most likely need read, append, and append to.

(3) Portals add additional filtering criteria to retrieve entitlements when you open a lookup form, Try to add entitlement for that customer/company, for which you are creating the case:

(4)Test:

For second question, it is related to access type for table permissions, you need set correct access type for account and contact Permission.

You can refer following link to know more information:
Configure security using table permissions - Power Apps | Microsoft Docs
Regards,
Leah Ju
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