Here are my answers to your questions:
- (1) How do they verify employment credentials?
You are required to provide contact details of your sources of income. They are not interested in the company that you work for. If the company is no longer in business, then you should state that in your application.
- (2) What precise exchange rate will they use. On the guidance, it says that they'll use, "closing spot exchange rate" from OANDA. But the OANDA website gives the "Interbank rate". Are those two interchangeable?
The Spot Rate and LIBOR (Interbank Rate) are synonimous.
- (3) If I get rejected for Tier 1 General, can I still apply for the Post study visa?
Don't know about this, sorry.