Always nice to see someone new having a go at creating their own threats, welcome aboard.
Tunnel Warden
1: Defences
It’s a nice standard creature, I would probably rate the defences as:
AC 21; Fortitude 18; Reflex 16; Will 16
I think that a Reflex and Will defence of 13 is probably too much on the low side for a level 5 soldier.
2: Spear Thrust
An attack that targets NADs instead of AC is usually lower, I think this should be:
Reach 2; attack +10 vs. Reflex;
I also see that you are using the normal damage expression, it is usual for soldier type creatures to use the low expression damage, but like all things it is a matter of balance. In this case as there is a little control and the creature already has a standard attack that uses medium instead of low I would go with
1d8+4 damage; on hit slide target 1 square
Tunnel Warden Shield Bearer
1: Defences
You are really playing around with the figures here, AC 23 is pretty strong for a level 5 creature (it is the equivalent of a level 9 skirmisher), especially if you start granting cover and superior cover. My prediction is that your melee characters are going to have a tough time hitting.
In all fairness the Shield Bearer should have the same defences as the standard Tunnel Warden. Now as these are themed creatures you can play with them a little, going from your slight deviation from expected damage I might suggest dropping the AC of the Tunnel Warden by 1 and increasing the Will of the Shield Bearer by 1, I might also increase the advantage of Stand Your Ground on the Shield Bearer by 1 (so that it can resist 2 squares of forced movement).
Another thing you might consider is creature version of the player feat that cancels the +2 attack bonus from flanking. This would fit thematically with the shield bearers.
2: Shield Bearer
Its an interesting idea. Like others have said I would change the mechanic to grant the bonus to all adjacent allies, its clearer and easier to arbitrate mid combat.
As far as I am aware there is no tower shield in 4e so you are setting a precedent by using it. Is the cover provided meant to be verses just ranged attacks? Or are you working under the principle that it would include melee attacks as well?
Really I would move away from having it grant cover and have it grant a shield bonus instead (your Tunnel Wardens don’t use a shield so it would work well). I might have the bonus at +1 to AC and Reflex, then +2 to AC and Reflex, I also might restrict the bonus to just allies and not the Shield Bearer itself. (If you felt that the Shield Bearer really needed the increased AC then I would just add 1 to AC defence, but again I would be cautious of pushing up a soldiers AC)
I might write it as:
Shield Bearer
If on its turn Shield Bearer moves at half or less than its speed it grants all adjacent allies a +1 shield bonus to their AC and Reflex defences until the start of its next turn; if the Shield Bearer does not move on its turn it grants all adjacent allies a +2 shield bonus to their AC and Reflex defences until the start of its next turn.
Anyway these are my initial thoughts, I hope they help.