Simple ROI is total return regardless of time. CAGR (compound annual growth rate) is the equivalent yearly return that produces the same final value — the right metric for comparing investments held for different periods.

No — ROI is nominal by default. Subtract the inflation rate over the holding period for real (purchasing-power) ROI.

Add reinvested dividends to the final value before computing ROI. If you took distributions as cash, add them to the realised gain.

Yes — any loss produces a negative ROI. -50% means you lost half your investment.

It depends on the asset class. Public equities average ~7% real annually; private deals are usually targeted higher to compensate for illiquidity. A 'good' ROI beats the alternative you'd otherwise pick.