To get the Character Set to run the Mapped Key, you can set it on this property.
As for your other issues. I think it is probably a matter of timing, and things are all happening at the same time, and potentially tripping over each other.
1. I would create a DXNothing ATG and put your dxnothing characters in it, then change the initial target for the videofx creation to be "(all&~DXNothing)#1". In fact, I would actually be inclined to just target is1, is2, is3 etc directly. Then make sure the first 5 slots are always your characters, and slots 6+ are the dxnothings.
2. Separate out the actions into different mapped keys.. i.e. the Create all the Sources, Create Viewers (you will need 1 of these for each slot as you only want the 4 viewers at a time and with no holes), Toggle Viewers.
3. Create some mapped keys that need to call the previously mapped keys with Do Mapped Key Actions, at specific times when you need them. i.e. When you activate your character set, you want to Create the Sources only. When your slots activate these can create the viewers and toggle the status as necessary** (you can set a Perform this Mapped Key action on each slot - although you may need to add on to an existing one here).
For 3. I do a couple of things and take advantage of some variables in ISBoxer.
1. I have a mapped key called Activate Character Set. In this mapped key are a bunch of Do Mapped Key Actions, which call other mapped keys that setup stuff for me. Like VideoFX Sources.
2. I have a mapped key called Activate Slot. This is set to run on each slot in a character set. It in turn runs a bunch of Do Mapped Key actions. 1 of which is Set VideoFX Viewers {SLOT}. This {SLOT} variable is replaced by ISBoxer with the slot number, so I actually have 6 other mapped keys called Set VideoFX Viewers 1, Set VideoFX Viewers 2, Set VideoFX Viewers 3,, etc..
Note: this was an image I mocked a while ago to show the place to set mapped keys per slot, so it doesn't show the Activate Slot as described.