Applying VOA after arriving in Hainan

No, at the time being this VOA is just a plan or better to say in some kind of a test phase. In other words, you should not rely on it.

I would suggest you to rely on 144 hour visa free transit. You can fly directly from Singapore to Xi'an and after your stay of maximum 6 days, you can return, but by taking a flight to Kuala Lumpur or Hong Kong or similar... You can combine the details, but it is important to have in mind that the immediate point before and after Xi'an have to be in different country or eligible territory (eligible territories: Hong Kong, Taiwan and Macau).