Mike,
The reponses to your problem so far have been very
good, let me add my two cents...
I agree that the age of the concrete isn't relevant, only
I'd add that it is a benefit to you that the concrete has
cured that long....most settling and movement of a slab
occurs in the first few years, so yours probably won't
crack or shear tile down the road.

The thinset is still "young", and will certainly release the
tiles more easily than normal. They could get very lucky
and the thinset could have bonded to the floor more than
to the porcelain, but even small amounts of thinset on the
tiles will make it very hard to tile the floor evenly. If they
are re-using the tile they'll have to get them very clean.

You ask "if they can remove the tile without damage [thinset
releasing easily I assume you mean], does that mean they
didn't put enough thinset....."
Porcelain requires the highest grade of thinset because it
it much denser than ordinary ceramic tile. If you see
bags of thinset around, read the bag and make sure it says
it can be used for porcelain tile.

You can visually check for uneven tiles by putting your
head low to the floor and look across. Porcelain tiles are
usually calibrated to be very true in size and flatness, so
the lippage should be minor.

I've been in malls or restaurants and said to my friends,
"look how absolutely terrible this tile job is". They'll look
and usually say, "looks good to me".
So....how much unevenness should you accept is hard
to answer. The general rule is can you stub your toe
on it with ordinary shoes or boots. My guess is more than
1/16" is not too professional, with this high quality porcelain
tile.

Bottom line is that the larger the tile, the more challenging
it is to get them flat. Are your requiring a tiny grout joint?
I won't use less than a 3/16" joint on a thinset job. If people
want less than that I tell them to get a marble setter to set
the tiles in a 1" mud bed, and expect to pay double. With
thinset though, you should expect a pretty flat floor.

What I fear for you though is this:
If they didn't do it right the first time....I don't know how they'll
be able do do it right the second time. Maybe your
contractor will come up with a better installer for you.

thetiler