Finished the UI logic and have a blue highlight for the currently selected slot. Only shows when while the Inv_UI is out (while selecting).
Releasing selection with an item in-hand, successfully stores that item in the specifically selected slot.
Also my impatient ass got a jury rigged function of the opposite when the hand is empty, grabbing the inv slot's item ID, spawning a new item of said actor class, and immediately attaching it to the player's hand. As if it was the same item stored before. Its buggy as hell and basically doesn't work, but I got the knife to spawn in my hand after storing it at least once before the night was out woo.
UE5 First person dishonored-like stealth navigation and combat.
Round out basic mechanics and systems to then build a robust and polished level design to play around it.