I absolutely agree Fusilli with your point. In fact, the first time someone said that the Slann had to accept the challenge or be sent back to the back row at a GT, I was shocked because I had always assumed in the old book anyway, that the Slann could not be moved to the back row and, thus, could not suffer the loss of inspiring presence and BSB boosts from the slann due to "retiring in ignomy."
Here is the problem: the new army book says that the Slann must be put in the second rank when it joins the TG unit (or another unit) with at least 5 models with the Guardians special rule but it does not explicitly say that the Slann must then stay there and cannot be moved to another rank. (see p. 35) I think this is an example of GW writing a rule that seems clear but not thinking about the language as written carefully enough and not anticipating the challenge rules (or maybe this omission was intentional). I think the intent is that the Slann must stay in the second rank until less than 5 Guardians models remain in the unit. The Army Book says that the Slann must move the the front rank of the unit once there are less than 5 Guardain special rule models in the unit. It does not say that the Slann may decline challenges and be immune to the loss of inspring presence or BSB benefits or immune to be reqjuired to "retire in ignominy" if the unit declines the challnege. Note also that the Slann always gets and passes the look out sir roll if the Slann is in contact with at least one model with the Guardians special rule., which means the Slann gets a look out sir even if one or two Guardians models are in its unit.
Go read page 102 of the BRB where it says that "if no enemy charcter steps forward to meet the challnege [or a champion], one of them [characters] must retire in ignominy." "The retiring character slinks off to the back ranks and is not allowed to attack that round-move the model into a rank where he's not in base contact with the enemy." "Furthermore, the model's Leadership cannot be used for any Leadership tests that take place that turn."
Notice that the rank of the character chosen for ignominy need not be the last rank, only a "back rank" that is "not in base contact with the enemy." This if often overlooked and can mean that, if charged from the rear, the character is not placed in the rear rank if possible and must accept the challenge if not possible. If the Slann must stay in the second rank of the unit and declines the challnege, that constitutes a "back rank" even if not the last rank. So, one can have a Slann decline a challenge and no other model accept the challenge and have it not move out of the second rank and still suffer the ignominy consequences. This was not a big deal for LD 8 cold blooded TG units as long as the BSB re-roll was still allowed, but a more recent FAQ in the last year clarified that the loss of Leadership of the character suffering ignominy includes loss of the BSB re-roll LD tests benefit.
Also, on page 102 of the BRB, two opposing models in a challenge (involving two opposing units that are in base to base contact/combat with each other) still fight the challenge even if it "is not possible" to bring those two models in the challenge into base contact. Thus, even if the Slann must stay in the second rank (which I think is the correct reading of the Guardians special rule) as long as 5 or more Guardains models remain in the unit, the fighting challenges rule allow for the Slann to accept and fight challenges from the second rank. (at least that is how it is now played)
The bottom line is that, under RAW, the Slann may be chosen to suffer ignominy when its unit declines a challenge unless GW issues an FAQ or rule saying that either: the Slann or its unit may decline challenges without teh Slann suffering the consequences of retiring in ignominy, or that a Slann cannot be chosen to retire in ignominy when the Slann's unit still has 5 or more Guardians models in the unit.