質問/回答例

Use Cross Object Formulas as workaround for Email Template processing limitations

With the cross object formulas, you can create a field on the object that references the associated object.
The email template can then reference that field on the primary object instead of an entirely different object.

Link

いつも回答を横取りする、その対策。
Email Merge Fields

Merge fields on email templates for CPQ

Use Cross Object Formulas as workaround for Email Template processing limitations

With the cross object formulas, you can create a field on the object that references the associated object.
The email template can then reference that field on the primary object instead of an entirely different object.

Link

{!SBQQ__Quote__r.SBQQ__Key__c}
{!SBQQ__Quote__r.SBQQ__NetAmount__c)
{!SBQQ__Quote__r.Total_Discount__c}

Classic Email Template: Case Merge Fields

as a workaround, you could consider putting formula fields on your case object that reference Account and Contact to display the names. That way you're not asking your template do the cross object reference, you'll be pulling all three fields from Case. Clunky, but should be effective.

Cross-object Merge Fields Do Not Work in Email Templates
https://help.salesforce.com/s/articleView?id=000385133&type=1

取引先責任者レコード画面からメール作成をする際に差し込み項目の挿入で取引先を選択しようとしてのですが、選択項目にありませんでした。下記のヘルプページに記載の通り、参照項目は差し込み項目で選択は不可なのでしょうか？

参照項目のメールでの差し込みは対応できていないと思います。
数式項目を使うしかないと思います。

メールテンプレートのクロスオブジェクト差し込み項目を取得できない
https://help.salesforce.com/s/articleView?id=000385133&type=1