I'd be glad to give you ideas on what's the best thing to do to handle your invoice transaction in QuickBooks Online.
For your first question, we're unable to create an invoice using an account type. Instead, set up a product or service and add the specific account you want to use.
Click the +New option. Then select Invoice.
From the Customer ▼ dropdown, select a customer.
Select the due date.
From the Product/Service column, select an account type instead.
Enter a quantity, rate, and change amount if needed.
Select a Class option.
Select Save and send.
Also, these are my suggestions for your second question. There are many options to do this but as you've described, this would be the best option I can offer.
You can create a list of parent customers who you create on behalf of. Under the appropriate parent customers, create sub-customers; the name of the people from who you're collecting the money.
Now create an invoice for the sub-customer. This time, you can consult your Accountant for proper guidance about the right account type and item name that you'll gonna use.
Instead of going to an income account, the bill that was created will go to a liability account and will show up on your balance sheet as a liability. Liability will reduce by paying your parent customer.
Please feel free to read these handy articles for your reference: