I have a visa to China - multiple entry, duration of each stay 90 days, issue date May 11, 2015, enter before May 11, 2016. On many websites I saw a description of this type of visa that I can live in China the whole year, just go out 4 or more times. But I think, I can go to China the last time on May 11, 2016 and live 90 more days. It's not really clear how it works, but I had a visa before - one entry, duration of stay 30 days, issue date March 23, 2015, enter before April 23, 2015, and I came to China on April 1 and got out on May 1. So I think the same logic should work for multiple entry visa. So is it one year for entry, but 15 months maximum for staying?
You can enter China up to the date the visa expires, so in theory making five stays of 90 days possible.
However you do need to be wary of the potential for Immigration deciding that you are attempting to live in China without the correct visa, especially if the time between exit/re-entry is only a day or two.
Folks with a business visa are less likely to be questioned about this as work related visits often require long stays. But doing this with a tourist visa may lead to secondary questioning after a couple of entries.
Yes, you can come to China on the day before visa expires. Multiple entry visa means you can enter China multiple times during the period of validity. 90 days for duration means that 15 months maximum for staying is available.