Yes, you can switch from one Xero organization to another within your ThaiTax account
No, you can book in either system, transactions will be synced in real time between ThaiTax and Xero.
Historical data will be synced when you sign-up to ThaiTax. Afterwards, data of the current accounting period will automatically sync with Xero (both ways), and you will have the option to manually sync historical data again in case you have updated any previous periods records.
Yes, ThaiTax is specifically designed to isolate undue VAT (input & output) from tax liability.
Yes, ThaiTax supports multiple VAT rates and all withholding tax rates following the various categories of services.
No, you can have an unlimited number of users, as long as all have access to the related Xero account