Well, looking at it an extra time or two, I think it's more a case of Chicken-Egg here. (Egg comes first, laid by a creature that's not a chicken, for those who don't know) In terms of practicality, I'd say first you place the model, then you evaluate if the position causes the model to fall or get stuck. If the model is standing on a 30cm wide cliff ledge and gets a bike from Booty, placing it inside the wall causes it to go IMM-2 while placing it off the edge causes it to fall (and applying what you wrote on minimum distance, it would be minimum distance as far as the centre of the new 50cm wide base is concerned). And last, but not least, you're not allowed to position the ABH off the ledge or inside the wall if you can place it without either going IMM2 or falling (i.e. if the ABH is standing in the centre of a 50cm wide ledge, you can't place the bike along the circumference of the old 25mm base in order to get the model to fall to a subjectively better spot below)
Are you or are you not attempting to apply the http://infinitythewiki.com/en/Models_and_Markers_Replacement rules? Because those rules certainly give the player permission to cause their model to fall if they want it to for some bizarre reason.
I can see how you read that, but I don't agree with that interpretation of "if there is not enough etcetc" which I interpret as referring to the space in which the model could be placed - not the space in which the model was placed.
