There are two fire exits in that area on the Southbound platform at least.
Where do they lead?
Its obvious if you've just arrived from Paris and want a southbound
taxi...

Is there any passive provision for connecting to the Chelsea-Hackney
line? Passive provision for the Chelsea-Hackney line seems to be built
into designs for some of the other stations, like the changes about to
happen to the tube at Victoria, and the ones that have already
happened at Angel (that's presumably why the cross passages there are
so wide and have very wide fire exit doors in the middle of them).

Its so cramped at Kings Cross St Pancras that I can only think of two
places for the Chelsea-Hackney to fit, unless they place it at some
extremely deep level. It basically heads in the same direction as the
northern line round there (although "northbound" and "southbound"
point in opposite directions); it either has to go parallel at the
south of the northern line, or to the north of it. To the south would
be so far away from the other exits and entrances that it would be
ridiculous connecting to it, so the north is the only sensible place.

That puts it roughly under the Great Northern Hotel - extremely
convenient for the Northern Ticket hall's passageways (suspiciously
convenient in fact). It also means that it heads under St Pancras -
the southern end of Thameslink would be very convenient for placing
escalators down to the Chelsea-Hackney platforms. They could add
escalators going in the other direction as well to link to the western
ticket hall, perhaps emerging in the now very wide passage between the
eastbound and westbound circle/metropolitan/hammersmith&city
platforms; that way you could get fairly directly from Thameslink into
the tube station.