Why are you getting a credit card in the first place ?
Because unless you know the merchant and he offers you a cash discount, fees of the credit card is already built into the product price. So, either you are subsidizing purchase of others, or getting the fees + some benefits back via your credit card.
An example, on Amazon buying a Mac Book Pro worth 2.2L will either give you a 10K cash back via Amazon Pay Credit Card, or you end up paying even that to Amazon subsidizing others. Similar math can be seen across stores.
@OP
1) Some banks allow you to have a temporary increased credit limit, OR, prepay the excess amount to the card and then make the spend. Do note you may incur a small fee + GST.
2) I would suggest no more than 2K.
3) Amount is always reduced from your limit, never from your account. Your account and your card are two different entities. As far as possible avoid physical swiping of the card, card cloners can duplicate the card and cause problems, tap to pay, chip and pin are better.
I use Amex with merchants I don't know because of their good purchase protection.
4) Each bank has a different process. Getting another card while you have a a relationship is usually easier, if not, close, wait, apply.