Lightly searing one's steaks on the outside is hardly equivalent to mixing raw and cooked, since it's mostly raw. By mixing, I mean something more susbantial, like having whole, thoroughly grilled chicken legs mixed with raw wild hare legs, or whatever.
As regards mixing raw and cooked re digestion-time, if, say, someone eats a cooked meal, and then quickly follows it up with a raw meal, then that can cause problems, though, logically, not so much the other way round, if you see what I mean. I have experienced this myself:- If I eat too much of a cooked meal, and then follow it up with a raw meal, then I often feel the driving urge to vomit, as the raw food seems to get blocked by the cooked food still being digested. I don't get this effect, admittedly, if the amount of cooked food I eat at the same time is small, though.
As regards extra digestive acids for cooked foods, that can be a problem for those with compromised digestive systems. For example, by the time I first switched to a raw, palaeolithic diet, I had developed extremely painful stomach-aches after eating any cooked animal food whatsoever. However, when I ate any raw animal food, my stomach-pains/ulcers did not register at all.