Small Property Management Business Seeks Accounting Advice

My son and I have five properties we rent out. Four are in San Francisco, one in London. We currently maintain our P&L on Google Sheets. We don't maintain a balance sheet. We store documents on Google Drive, and use Google Docs for word processing. We need ongoing advice on various issues, including the ones listed below. Periodic Reporting: * End of year processing * Preparation and submission of tax returns Maintaining Independence of Accounts: * What is the best way to separate the accounts from our personal accounts? We don't want the overhead of setting up a separate corporation * Provide for easy transfer of management annually between myself and my son. The idea is that he's in charge one year, then me the next, and so on Bookkeeping: * What accounting and bookkeeping tasks should we give to our administrative colleague Sara? * What is the best way to train Sara? Audit & Security: * How should we incorporate checks and balances, to reduce the chance that money is stolen or inadvertently misplaced? * How should we maintain our simple online systems so that money is unlikely to be stolen through malware exploits? * How should we back up our Google Docs-based environment? * What is the best way to check the data is valid? * Should we start double-entry bookkeeping, and have a balance sheet? We want to keep the system simple and easy to maintain International Taxation: * Should we retrospectively apply capital costs to the London property, amortize the depreciation, and send in adjusted tax returns? * How should we make the London reporting consistent with US reporting, for tax purposes? * I am resident both in San Francisco and London. I currently submit US tax returns. I am joint resident, and could starting submitting UK tax returns. Would it be better to submit UK rather than US tax returns? Miscellaneous: * What key management statistics should we review every month? * How should we best incorporate car port rental income?