Or you mean vaisnavas inserted all of these verses, not just tweaked a few words here and there?

Yep. The obvious answer is that

a) We can tell interpolation by comparing manuscripts and language

b) If those verses are accurate, then Lord Shiva went through a ridiculously complicated plan to achieve very little.

c) Since the purana misunderstands Advaita, either the verse is interpolated, or Lord Shiva misunderstood Advaita. Which is more likely?

Well, according to quoted verses it WAS Shiva's intention - to mislead people of Kali yuga. Or do you think it was an oops moment on his part and he had no idea how his teachings would be interpreted later on? Or do you think Shankara wasn't an incarnation of Shiva at all but just some random dude with above average intelligence?

Well, it's highly unlikely that Lord Shiva would mislead people for no good reason. Also see the reply above. I think Shankara was a genius, but the claims of incarnation were never made by him. He was certainly human.

Our view is that he was the worst thing to happen to India.

You can keep your views and anti-secular crap to yourself. The rest of the world acknowledges his greatness.

Ramakrishna Mission has chicken farms as integral part of their franchise, afaik.

Evidence? This does not mean they eat chicken, either way.

save for brief period of poplarity Abhinavagupta was never even close to Shankara.

Popularity does not denote stature, specially not google hits. By google hits, Prabhupada is the greatest Hindu ever, which he clearly isn't. Read more on Abhinavagaupta and his works and you'll realise why he is so great. The GV aesthetics is lifted straight from his works, for one example.

I only said that when talking about Shankara one needs to keep in mind quoted material.

Yes, and so far I see nothing quoted from actual works of Shankara, just purana verses which misunderstand Advaita.