What percentage of merchant Bitcoin receipts are immediately converted into fiat? Does anyone have access to this data from Bitpay or Coinbase perhaps? 

If so please break down the data by country and industry if available and how these statistics have changed over time.
The last stats Bitpay published were in December, 2014, so it may be somewhat out of date. They said that 10% of the merchants keep settle entirely in Bitcoin, 41% of the merchants keep a part of their settlement in Bitcoin, and the remainder settle entirely in fiat.

That said, there are plenty of instances where BitPay merchants get settled in Bitcoin, and then use a local exchange to turn that into fiat. This is in instances where, for example, BitPay doesn't support fiat payouts in that country, or BitPay's fiat rate is lower than the local exchange's. I think it's safe to say that the actual percentage is much lower than 10%.

Of the 41% that partly settle in Bitcoin it's hard to know what the average percentage split looks like, and BitPay don't make that information available.