Depends what visa and what country
Are you seeking a visitor visa for the UK?
Yes, you can apply for a Canada visitor visa from the USA.
Do you intend to apply for a family visitor visa to the UK?
Of course not. You cannot work in Singapore with a visitor's visa. This would be illegal. You have to apply for a Singapore work visa.
Yes, a return ticket is typically required for a US visitor visa to show that the visitor plans to leave the country before their visa expires.
Do you have the required consulate letter for your visitor visa application?
Yes, Indian parents can apply for a visitor visa to the USA.
Nobody can work in the visa on a visitor visa. Short duration courses like cookery class that will not add any credits to a course can be attended using a visitor visa. But working in the US, that will enable you to draw salary is not allowed on a visitor visa.
Have you provided all required documents to the consulate for the visitor visa application?
The maximum stay allowed for a US visitor visa is typically 6 months.
As a visitor on a visa, you can typically stay in the USA for up to six months. It is important to adhere to the terms of your visa and not overstay your allowed time.
If for some reason you are stopped by border security, your visitor's visa will not take precedence.