You can undo a settlement simply by tapping the red undo icon on the settlements screen. This will add the settlement amount back to the debt updating you balance accordingly. Note that if you have already paid the person (for example via Venmo, cash, google pay, cashapp or some other means) you would have to undo that as well. SplitPal cannot automatically cancel the transfer as it took place outside of SplitPal app.
Absolutely. SplitPal has built-in currency converter with up-to-date exchange rates.