Yes, you may purchase the tourist card in advance. You will need to present a copy (print out) of your (return) flight ticket and other required travel documents.
You could apply for an e-tourist card via https://suriname.vfsevisa.com. You would have to pay with credit card.
No, you cannot obtain a visa at the Schiphol in Amsterdam or at the J.A. Pengel airport in Suriname. A visa is not the same as a tourist card. In the Netherlands, the visa (sticker) can be obtained at the Consulate of Suriname in Amsterdam or via an online application: https://suriname.vfsevisa.com (Note: You would have to pay with credit card.)
No, to purchase a Tourist card it is required to submit an actual flight ticket possibly an e-mail ticket.
As such the Tourist card does not have a validity. The Tourist card can be used only once. This means that it allows a single entry into Suriname, after which it has no use. On arrival in Suriname the length of stay is stamped in your passport.
Upon arrival the Immigration will issue a stamp for a Short term stay of 30 days. Persons of Surinamese origin may after they has entered with a Tourist card or visa stay in Suriname for a maximum of 6 (six) months.
Persons of non-Surinamese origin are allowed a stay of maximum 90 days.
If one intends to stay in Suriname after expiration of the Short term stay of 30 days, it is required to register with the Department of Alien Affairs of the Ministry of Justice and Police, submitting the original passport and flight ticket, for an extended stay in Suriname as mentioned above.
No, this is not possible at the border. A Tourist card can be bought at the Embassy of Suriname in Georgetown (capital of Guyana).Or you could apply for an e-visa through https://suriname.vfsevisa.com (Note: You would have to pay with credit card.)
No, is it not allowed to travel on a Tourist card for that purpose. In your case the Voluntary work is considered the main purpose, while the Tourist card may be solely used for strictly tourism purposes.
No, it is not allowed because of your internship. You will need to obtain a visa at the Consulate of Suriname in Amsterdam.