What's so absurd about running Pursuit and Low Kick? It's not an illegal combination and has its uses. If you run Pursuit only, you're still screwing Shedinja up, meaning you have just wasted one pokémon slot to (maybe) get a free Head Smash, that might miss or be walled. Also, considering something 2/3 of the common leads have means of defeating it, that means the whole concept of "a lead should put you in a situation of advantage as many times as possible" is taken for granted.
Really, I'm never going to setup at the face of Shedinja if I'm not 101% sure I can deal with it no matter how many SDs it get, and I'm pretty sure not many players will as well (at least those who have an idea of what they're doing).
Ambipom isn't going to risk a burn from Shedinja, right? If it tries to Pursuit, Focus Sash kicks in, and Shedinja uses Will-o-Wisp. Now you have a near useless Ambipom unless you have a Heal Bell user or something.
And if Shedinja SD and Ambipom Pursuits, again Focus Sash saves you, and Sucker Punch finishes Ambipom unless you predict and switch out.
Or if Shedinja uses X-Scissor then Sucker Punch...
I'm too lazy to run some calcs, but i think Shedinja has a chance to KO Ambipom with a SD Sucker Punch or a X-Scissor followed by Sucker Punch. If not... well...
So, it's more like a prediction war where Shedinja has the edge unless Ambipom has Taunt or Shedinja doesn't attack on the first turn.
It's still easier to use a Dugtrio lead against Ambipom logically.
About Ambipom attacks, Pursuit and Low Kick together means:
1- No Fake Out( no Fake Out Ambipom? wat)
2- No U-Turn
3- No strong STAB move
Without U-Turn or Fake Out, Ambipom isn't going to do it's main job as a lead (racking up some free damage). Without a STAB move, it's going to be walled by things it wasn't meant to be walled.
You can use a Fake Out/Return/Pursuit/Low Kick Ambipom for a "universal" coverage, but no U-Turn on a leading Ambipom seems... weird.
Mind you, i'm talking about the lead Ambipom. I'm sure almost every lead Ambipom runs U-Turn.
You can use Trick Shedinja too on the lead position. No need for SD, and you probably crippled something(or not).
What i'm saying is that Shedinja needs too much support to work normally while on the lead position it doesn't, despite being a good lead or not.
Like you said, Shedinja is easily "wallable" or easy to dispose of if you know what you're doing(and sometimes even when you don't know what you're doing)
Which is why i think Shedinja is only "acceptable". It's not that good as a lead IMO. Like i said, i wouldn't use it in a team, unless i have some time to create those weird teams...