I’d recommend reviewing the “Basic CSV” workflow in the Tiller Community Solutions Add-on - Import CSV Line Items workflow as a starting point. That workflow can help structure and implement CSV data migrations.
As for your other questions… You can sync both business and personal data into a single spreadsheet, but data will be consolidated into the core sheets— Transactions, Categories, Balance History, etc. You cannot plumb linked accounts to discrete versions of these core sheets. If that is your goal, I’d recommend separate spreadsheets.