I cap into the accessory groove quite often. I learned how to do it from a window manufacturers rep. If you take your snips and cut into your metal just a bit and twist, it makes a little hook which will hold into the groove, kind of like when you punch out the siding to fit into your undersill. It is not my preferred method, but it has it's place. I by far would rather use the sill extender, but with casements, awnings, and anything with a mull, they rarely work out, and one manufacturer doesn't even supply them, so we have to either put down a 1 X and cap that, or cap into the groove.
As far as your metal not being up against the wood, or hollow capping as we call it, I was always against it for years, because if something hit it, the metal would dent, then I realized that it still dents whether or not there is wood behind it. Which is why I always try to talk my customers into not wrapping the wood threshold of an entry door, but to paint it. Within 6 months of wrapping a threshold, people have kicked the, well, lets just say it is a high traffic area that tends to get beat up.
The screw in the middle was probably to straighten out a bowed master frame. Sometimes the top rail will smile at you a bit, especially on wider units.
I'm sure they are plenty of people who will tell me I"m wrong, or this guy's a hack. But....somebody must think it's OK because I'm still working.... Knock on wood.
Opinions and something else,