Not so much interesting for Daemons imo, but here's some factoids that may be worth some discussion:

1) Mindshackle Scarabs if affecting a single model can cause that model to direct attacks against itself.
That's... wicked. I guess that means we'll have to multi-assault things into units if we want to take out something that has a Necron Lord or something in it so that we drastically increase our chance to actually do stuff. Against us Daemons, I think it royally screws us due to how many Monstrous Creatures we have, seems to me that you could always try to Soul Grinder assault a Necron lord that you know or suspect to have Mindshackle.

2) A Necron Lord can not attempt it's Reanimation Protocols if the model has been 'replaced' by something else such as a Chaos Spawn or has been transformed into Trazyn.
Have Tzeentch Daemon Princes moving up in close support to primary combat blocks such as Greater Daemons and attempt to snipe the Lords beforehand with Boon?

1) We had quite an in-depth discussion on Mindshackle scarabs not that long ago. I suppose the fact the models attack themselves now in the right circumstances just confirms they are as nasty as we thought they were. Soul Grinders or just swamping them with Bloodletters were the best ways we could think of at the time to deal with them.

2) You could try it, but the fact Lords and Overlords are T5 means there is only a 1/6 chance of them failing so I'd try and find another means of dealing with them. Boon could be useful though for dealing with Wraiths or Crypteks who are T4 but equally annoying to take care of.

1) Yeah, multi-assaulting is key vs MS scarabs. Get any model other than your big stuff into b2b with the guy with scarabs and you don't have to worry about them. The key is to just allow your uber units to attack, cuz you'll clean right through necrons and likely end up sweeping them.

2) boon is neat but I don't like the 5+ needed to actually succeed. It's a nice side effect but I don't typically like boon.

Some other rullings of note are the changes to other armies. Like the eldar farseer and tyranid shadows of the warp now have you 'roll an extra dice' on psychic tests instead of 'roll 3d6'. So they likely stack now. That doesn't really matter to demon armies, but other armies may not like that much.

Actually, boon may be helpful vs annoying units of scarabs. Then again, daemons are one of the few armies who really shouldn't be afraid of scarabs in the first place...

I love the idea of Boon of Mutation but by the gods it's so useless...

As for Mindshackle scarabs, thats how we interpreted the rule here too. It makes sense, I guess. Better the thirster hitting itself than squashing your own bloodletters...

A crazy idea I had for maximizing the Boon chance was to have a Lord of change with it, Kairos and 3 DPoT with it as well. All hitting on 2's, and with 5 dice being thrown there's a reasonable chance of you rolling at least one 6, although granted it'll be all against this one Character (but chances are imo if it has Mindshackle it's also going to have Tesseract Labyrinth). The huge flaw with this idea I guess is because you've devoted so many damn points into it that if you put everything within the 6" of the Necron Lord in order to snipe (which would surely require some clever ass positioning) then you'll have to assault otherwise because well, better you getting the extra attack than the Necron's. This may mean that spending those 180points(!) on wings may be necessary :(

Lord of Changes will be hitting on 3's against most things in the Necron armylist (short of Varguard Oberyon or whatever) and wounding on 3's against the gamut of T5 things. Alas. Then again, with you assaulting though you'll get to chose which things are going to be in b2b with the Necron.

The best/worst thing about the 'unit' set up would be though that they're all seperate targets (because they're Monstrous Creatures), which means they can't flow on wounds etc any that are successfully inflicted (for example; they couldn't say that of 10 Praetorian's with War-scythes that dealt 8 wounds they couldn't slay two Daemon Prince's if they weren't in base contact with two Daemon Princes and/or didn't get an allocated 4 hits/wounds on 2 of them each etc, etc)

I've never considered spamming Boon. That is an interesting idea. The power of boon is it reduces an enemies force and adds to yours. I wouldn't care so much about trying to overload the boon chances of affecting an unlikely target. I'd be more interested in targeting targets it would likely affect. Like in your example above you can drop all 5 of those boons on a unit of scarabs and net 2-3 spawns a turn. It may be worth a try :)