Hi kasden,
I wouldn't get too hung up on the year of birth from his death cert. as the informant may not have known the exact year of birth & as you have stated, he didn't always tell the truth or was maybe just forgetful as men are when it suits

You need to try looking on census without putting in any specific area.
It's even possible he was illegitimate, never christened but used his father's surname which was a common occurrence.
How can anyone give "any sign that the Findley's actually did come from Forfarshire as shown on Robert's marriage certificate".
There are many free sites to look at census records for an indication & I'm sure you will see the surname Findlay/Findley in many areas.
What were the names of his children in order?
Have you looked for Jane/Jean Hales although that surname looks to be very rare.
Robert may be with his mother under her surname of Hale/Hales?
OPR births in Scotland on SP have only 19 entries 1538 - 1854 for the whole of Scotland for Hales.
With "Surname beginning with" Hale there are only 132 entries (ditto) above
Annie