Can I start a 1041 fiduciary return using TurboTax and can I do this if I have already submitted 2018 tax returns for the months the decedent was alive (through August)?

In order to prepare a Fiduciary income tax return, Form 1041, you must use TurboTax Business.  It is only available on the desktop software (CD or Download).  https://turbotax.intuit.com/small-business-taxes/

Although TurboTax Business handles Form 1041 for trust and estate income tax returns, not all trusts and estates require this form to be filed.

A trust is a legal entity formed under state law, creating a relationship where one person holds title to property subject to some benefit to another person(s), referred to as a beneficiary. Trusts can be created for a living person or come into existence at a person's death.

Many trusts require the filing of a Tax Return for Estates and Trusts (Form 1041), but some don't. Here are some examples:

Decedent’s Estate
In the case of a death, the executor must file a Tax Return for Estates and Trusts (Form 1041) for a domestic estate that has:

• Gross income of $600 or more for the tax year, or
• A beneficiary who is a non-resident alien

Domestic Trust
You must file Form 1041 for a domestic trust that has:

• Any taxable income for the tax year
• Gross income of $600 or more (regardless of taxable income)
• A beneficiary who is a non-resident alien