Seems you got everything all "wrapped up". Such effective Trim certainly shouold always be a "staple" of any DIY Screen.
Now all that's left is for you to "nail" it down....literally. Use a Finish Nail sized Nail Set (...very small head...) to set the heads of the 1.5" bright Finish nails just into/below the Trim's "MDF" surface...which is of course below the Velvet Layer. This will effectively make the Finish Nails invisible.
Myself, often I will set / drive the nails in at a slight angle, which in turn minimizes the "view-ability of the nail heads from a front viewed perspective. This technique is a little trickier, and replicates the end results I get if I use a Finish Nailer and also hshoot nails in at angles. (I always go back and "set" then with a Nail Set no matter what...if I can in any way, from any angle see a vestige of Nail head showing.
Much ado over nothing? No Sir!
With Black Velvet being so....Black, any brighter spot on the surface of BV will stand out. Even "mushed down Velvet nap" will show up as a brighter area.
BTW...when Tapping in Nails, use a Hammer that has s "CLEAN & SMOOTH" Striking Head, and try to time your strikes so the last one made without contacting the Velvet full force set's the Nail at least Flush with the Velvet. Then let the Nail Set do the rest. Sometimes, a "too hard" strike can permanently Flatten the Velvet's Nap...causing a sheen spot.
Now during a showing in a completely darkened environment, BV Trim contributes nothing but effective edge masking. It is when one can watch "The Big Picture" with varying degrees of Ambient light and the BV's "Reference Black" helps "Fool the Eye" into seeing darker blacks on Screen that it's worthiness is accentuated. Obviously...with my own propensity toward DIY Screens that perform well in Ambient light, I want the BV's performance and cosmetic appeal as pristine and perfect as it possibly can be.
But frurgudness sake! Ya can't have the Heads of Finish Nails spottin' up the expanses of Blacker-than-Black Trim, or have obvious shiny flattened areas jumping out at your guests, ruining the "Oooohhhh...Ahhhhhh" appeal of the Black Velvet.
That would be bad.
It is by anyone's reasoning, a case of getting the maximum benefit of using Black Velvet...both performance &
cosmetic-wise. And like all aspects of DIY Screen building itself, you'll almost always get at minimum the end results being directly proportionate to the care and effort out-layed.
Better still... often when the total sum of several layers of thought - care - effort comes together it can produce results that are wholly unexpected and far better than expectations.....and that is for many the really most welcome & exciting time of all.
I've seen it happen many time over.
.......and it's always been just as exciting for me to watch it happen.