Here are the rules at play that I can see...
(
pg. 64), Issuing a Challenge
"Challenges are issued at the start of the Fight sub-phase, before any blows are struck..."
(
pg. 20), Fight Sub-phase
1. Choose a Combat
2. Fight Close Combat
3. Determine Assault Results
4. Repeat Fight Sub-phase or Finish Assault Phase
(
pg. 22), repeats the above in more detail over several pages
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2420315a_Necron_6th_Ed_V1.pdf
(bottom of
pg. 3 of 5)
"At the start of the Fight sub-phase, randomly select an enemy model in base contact with the bearer of the mindshackle scarabs. That model must immediately take a Leadership test on
3D6. If the test is passed, the mindshackle scarabs have no effect. If the test is failed, he inflicts D3 hits on his on unit (or himself, if on his own or in a challenge) when it is his turn to attack...."
Finally, note that Challenges can take place over multiple rounds of combat.
+ + + +
A literal reading of the rules might be that
=================================
Mindshackle Scarabs happens before you, "Choose a Combat" in the Fight Sub-phase, if it happens before you Choose a Combat.. well...
Challenges happen "At the start of the Fight Sub-phase, before blows are struck", which could be read as any time before the 2. Fight Close Combat step...
The comment in
MSS errata could relate to rounds 2+ only
My gut feeling
===========
That Challenges should be issued first, then
MSS should resolve. Unfortunately, that is not how the rules are written.
They should have wrote
MSS better in the
FAQ to say that it happens, "At the start of the Fight Sub-phase, before blows are struck and after Challenges have been accepted or denied."
This needs a further
FAQ update.