You are correct in your interpretation: the target's leadership is determined by the
Mob Rule special rule.
Ork Psychology and morale is directly linked to the number of Boyz
around them at any given time. ... Ork mobs may always choose to
substitute the number of Orks in their mob for their normal Leadership
Mob Rule! is in effect at all times, regardless of whose turn or what action is requesting the leadership value. Thus, 15 Ork Boyz means subtracting 15 from the 3d6 that the Psychic Shriek-attacking player rolls.
This is in line with the answer provided by @DavidCEllis.
To answer your bonus question, while the challengers may be separate from the assault in terms of wound allocation, they are still very much part of the unit/mob they entered the assault with. Therefore, the character in the challenge as well as the accompanying unit can use the Leadership value Mob Rule! bestows.
Rule-as-Written vs Rule-as-...Expected
Contrary to the BRB (6th and 7th), this answer asserts that leadership may exceed the normal 10:
All but one of the characteristics are rated on a scale from 0 to 10.
This seems to be commonly held as an unbreakable rule, and indeed forums about this at large does seem to interpret the rule with a hard capped 10. In support of this, the Ork FAQ for 6th edition actually partially addresses this:
Can a Weirdboyz leadership be increased to more than 10 by Mob Rule?
A. No, it can be increased, but only up to a maximum of 10.
Much to my dismay, however, we will find this rule almost impossible to reference for current play. GW has discontinued distribution of 6th edition FAQs to replace it with the 7th edition FAQ, where this rule is woefully absent.
In light its (deliberate? accidental?) removal from the FAQ, the rule from the BRB (again 6th and 7th) iterates:
On rare occasions, a conflict will arise between a rule in this
rulebook, and one printed in a codex. Where this occurs, the rule
printed in the codex or Army List Entry always takes precedence
It seems like GW could have easily cleared this up by adding to Mob Rule in the FAQ "to a limit of 10 LD" (even in the FAQ they directed this only to Weirdboyz) or by saying "superseding the normal limit of 10 LD" or any variety of phrasings... but RAW, they didn't.
This is likely to be a hugely unpopular interpretation, and thus most players (non-Ork, Ork?) may amicably decide that LD10 is the cap, which is fine and the intended way for rule conflicts to be resolved. That said, in 6th I'd say it's perfectly viable to do a roll-off for rule interpretation.
If every model in a unit has this special rule, and the unit fails a
Morale check or Pinning test (after any re-rolls they may have), roll
immediately on the following table:
The flavourful leadership boon is entirely gone, replaced with a d6 roll table, and it's pretty straightforward.