Hi, I'll take a stab at your tether anchor questions by checking in the LATCH manual.
The closest match to a 15-passenger Chevy Express for the '06 model year in the LATCH manual is only listed with a second and third row in my 2009 LATCH manual, so I think your van's owner's manual is likely more comprehensive once you get your copy of the owner's manual to verify tether anchor information. The '06 Savanna entry is similar to the Express entry with a bit more detail and also mentions a 4-passenger bench seat without tether anchors. I'll type up the Savanna's LATCH manual entry info. a bit later in my reply for you.
Here's what the LATCH manual entry states for the 2006 Express:
Front seat - tether anchor on base of seat frame
Second row center - shared tether anchor on back of seat
Second row outboard - 2 sets of LATCH with shared tether anchor on back of seat - I believe this means either the passenger side or the driver's side shares a tether anchor with the center seating position. Based on a note for earlier Express model years (2001-2005) and the '06 Savanna's entry that I'll type up in a moment, two carseats cannot be connected to same tether anchor at the same time, and this is likely true for the 2006 Express but there's no note expressly (oops, pardon the pun) stating that for the 2006 Express model in the LATCH manual.
Third row, 3 passenger seating, center- tether anchor on back of seat at bight line.
Third row outboard - no tether anchors
Based on this info., the van has 3 tether anchors total in the second and third rows, and 4 total if you also count the front passenger seat's tether anchor.
As promised earlier, here's what the LATCH manual states for the 2006 GMC Savanna, which might flesh out more details for your '06 Chevy Express van if they're equivalent models. (I think they are equivalent, but not 100% certain):
Second row center - shared tether anchor on back of seat, looks similar to seatbelt latchplate, use tether anchor for either center or outboard position. Two carseats cannot be attached to same tether anchor.
Second row outboard - 2 sets of LATCH
Third row center - tether anchor
Third row outboard - no tether anchors
The LATCH manual's entry for the '06 Savanna also lists this info. for a 4-passenger bench seat: Do not use carseat requiring a top tether anchor as no tether anchors exist for this seat.
On the retrofitting question, GM doesn't provide additional top tether anchor retofits in GM models that are already factory equipped with some top tether anchors. I don't know about your headrest or seatback height questions. Hopefully we have someone familiar here with your van to address that further for you. Congrats on your new vehicle.
