Wrecking ball: That's exactly what I wanted, a way to overcome the PL vs objects so that inertia won't be so easily stopped by simple walls.
Assuming an ~4 Inch brick wall, it would have Tough 7, for a 'routine' or 'average' result of 17. In order for it to be destroyed the DC of the check would have to be 17+15=32 Base 15+5(finishing attack)+10(PL)+5(Power Attack)=30. +2 from the Wrecking ball bonus is exactly what Inertia needs to be able to 'reliably' go through a standard 4" thick brick wall (Or equivalent).
Slam:
Action: Actually, increased action is page 148 in the 3e handbook.
Side Effect: I was under the impression that 'side effect' was equal in power to the effect, meaning she'd get hit by a str 7 Penetrating attack, not 11. (Still not sure how you got 11, even if she does take full damage counting str that would still only be 10).
I'm starting to think the easiest way to do it would be to make it effectively a 'ranged' attack, having the 'fluff' be that she's charging into them. Give a few small limiters that they have to be targets she could reach with a movement mode (Leaping/Speed)
*Also: I noticed that making it str based actually costs me MORE than buying a full rank damage effect, and that's not even counting the fact that I'm paying for str, b/c I have to pay full for each extra, but only get back for the flaws points equal to the actual damage rank.
[sblock=examples]
Inertial Slam: Damage 10, Ranged(10), Multiattack(10), Accurate, Side Effect: If any target successfully resists, Inertia takes damage(-10), Full Round Action(-10) Limiters: Must be able to reach all targets with movement mode(-2) - 9pp
Inertial Slam: Damage 7(Str based), Ranged(10), Multiattack(10), Accurate, Side Effect: If any target successfully resists, Inertia takes damage(-7), Full Round Action(-7) Limiters: Must be able to reach all targets with movement mode(-2) - 12pp
So the Str Based one costs me 3 more points PLUS the 6 points worth of str going to power it... that makes it essentially TWICE as expensive as the other.
[/sblock]
So I think this is what I'm going with.. at least until someone tells me what's wrong with this variant.

Inertial Slam: Damage 10, Ranged(10), Multiattack(10), Accurate, Side Effect: If any target successfully resists, Inertia takes damage(-10), Full Round Action(-10) Limiter: Must be able to reach all targets with a movement mode(-2) - 9pp
LEGACY:
Also were you ok with the proposed backstory I had for Inertia? He's not a 'direct' legacy, meaning his powers differ slightly from his parents, but b/c he had TWO super-powered parents, his power is a mixture of theirs (Brickish super str/toughness and Super-speed)