Hello April,
>> What if i want to make bulk changes to the core elements used within a group of templates?
You can't make changes to group of templates together.
What you can do is make changes in the structure layer of template and all the forms created from that template will get updated. [0]
>> For e.g. i would like to update a number of templates with a new logo
If your logo is part of structure layer of template. Then yes, updating template, will update all its forms.[0]
>> a revised T&C all in one go, including adding/deleting text or data fields in the templates? Can the above be achieved using fragments?
This can't be achieved by template, but this can be achieved by form fragments. All the forms which are using the fragment whose fields or T&C you updated, shall be updated automatically. [1]
Ps:
[0] https://helpx.adobe.com/aem-forms/6-2/template-editor.html
[1] https://helpx.adobe.com/aem-forms/6-2/adaptive-form-fragments.html