Honestly most of us are DIYers and suggest that if you are prepared to do deep research in the right places and read read read .. that you can do it yourself.
The only point from your summary that is going to catch you out .. visa service , immigration lawyer or DIY.. is Montreals strict policy of requiring the USC to be physically resident in the US prior to the interview. This usually requires the USC moving to the US once the interview date is set, so they can provide documentation of their permanent presence in the US Drivers licence, voting card, phone bills, bank statements etc. Many lawyers and visa services wouldn’t be aware of this specific requirement of one particular consulate.
She must file tax returns for at least the last 3 years or be able to explain why she did not need to file a US tax return.. eg zero income