Hi Partner,
1.There is an OOB way for your requirement. For example, I have contact subgrid on my custom entity form, and I want to create emails for contact in contact subgrid directly in custom entity form in portal.
Here are the steps.
a).Create a new MetaData on custom entity form to add contact subgrid.

b).Select the type to "Subgrid" and entity to "Contact".

c).Add a View actions as Create to allow user create new contact in custom entity form.

d).Click the "Advanced settings", and add a new Item Actions as "Create related record".

And select the relationship you need, here I chose email. And you should have created an entity form for creating email before.
And please make sure you have the permissions to all the entities.
e).Test


2.If you mean when you select an option from dropdown list, then there will be some fields hide or displayed dynamically by the option on the form? If so, you should add custom Jquery code to monitor the option you selected and do the actions.
You could add JS code in entity form Additional settings.

And you could get the fields and values easily by $("#fieldanme).val()..
Regards,
Leo