1. StageShow adds its own WordPress Capabilities which control access to the various admin pages. So you just need to create or modify the capabilities associated with the appropriate user. Section 9 in the user manual gives details.
2. StageShow collects payments in all currencies that I know are supported by PayPal, which are as follows:
Australian Dollars (AUD),Brazilian Real (BRL),Canadian Dollars (CAD),Czech Koruna (CZK),Danish Krone (DKK),Euros (EUR),Hong Kong Dollar (HKD),Hungarian Forint (HUF),Israeli Shekel (ILS),Mexican Peso (MXN),New Zealand Dollar (NZD),Norwegian Krone (NOK),Philippine Pesos (PHP),Polish Zloty (PLN),Pounds Sterling (GBP),Singapore Dollar (SGD),Swedish Krona (SEK),Swiss Franc (CHF),Taiwan New Dollars (TWD),Thai Baht (THB),U.S. Dollars (USD),Yen (JYP)
I believe that you can use PayPal to collect payments in INR which are then converted to USD, but I have no experience of using PayPal in this way, and don’t know if this would require an update to StageShow. There would, of course, be fees associated with the currency conversion.
3. Seating Layouts are fully customisable. For complex layouts some knowledge of HTML and CSS will be required. A description of customisation of Seating Layouts is in Section 6 of the user manual.
You can find the user manual here.