Let's say I mash in at 65 C and over an hour it drops to 60 C - will this have a detrimental effect on my brew?
The bulk of the starch-sugar conversion happens in the first 15 minutes of the mash, so that's probably not a big problem. You might need to mash for 90-120 minutes rather than 60 to get a really good, full conversion, though.
I usually have to do a longer mash when I'm mashing "cool" (which you are), anyway.
I have experimented with a couple overnight mashes. I mash in and hit my 154F mash temp, close the cooler and wrap it in few heavy towels and an old sleeping bag. I know that normally my cooler holds the 152-154F temp for at least 60minutes. So I am not worrying about conversion time. But in the morning my mash may be at 135F.
So to address flavor concerns there aren't any that I have noticed with a cooling mash. Conversion in your case is still a concern as you mashed in cool and lost temp pretty quickly. Outside of conversion, I wouldn't worry about flavor issues.