Not that complicated, no. Take a look at the “Hybrid” example map in Experimental. In this map the game switches between turn based top-down and real-time third person when the player enters a trigger or ends a combat. Instead of using a box trigger you can implement whatever code you want for an enemy spotting the player.
You can use abilities for this. Make sure to look through my ability system tutorials to understand this better. In all abilities there is a Server Hover event, which includes a reference to the tile being hovered over. Within this event you can check the GridUnits TMap in BP_GridManager for enemy units and run whatever code you want if this returns true.
There are many ways to do this, sure. For individual items you could create an inventory array in BP_Unit (or whatever derived blueprint you are using as a base for your own units), or create an inventory component similar to the ability system one. Or you could store it all in a data table. Depends on the requirements of your game, really. For storing stuff on the map you can take a look at the HydrasLair example map, where I do just this (using Add/RemoveObject as you suggest).
Looking forwad to your reply.
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This is done within the abilities. Check out BP_Ability at the PlayerActivate event to see how the ability searches for nearby units and display them. When the ability loops through these units and place markers you could add a check that looks up their elemental weakness enum and places a custom marker based on the value. You would probably want to do this by overriding some of the PlayerActivate code in a derived ability blueprint.