I've used a Mantis and yes, the dethatcher can be used to scratch the surface. I'd be OK with you doing that if you wanted to!

It'd be as simple as running it over the top once, quickly, and shouldn't take that long. Schedule about twice as much time as you think it should take, because you know how that goes.

Haha absolutely. However would you try and do it without the dethatchinf attachment. If I get the tiller I won’t be getting the dethatching attachment right away. should I try and use the tiller to expose the ground? I’d rake and flattened the ground before planting seed but is it going to be over kill to till an area before seeding?

I wouldn't till because you're just asking for trouble and uneven ground. If you just let the tiller skip over the top, you might be able to get the tiller attachment to scrape the surface a bit (particularly if you go the "wrong" "front" direction instead of pulling backward the way the Mantis digs).

But doing that does cause some skipping and misses. That's not really a problem when seeding, fortunately--the seed won't care if it's buried or not. I did no preparation and ended up with a great lawn.