The situation simply isn't covered by the rules. The likely answers which one might see include:
1. The outflanking unit is destroyed, crushed under the weight of the contradictions between 'It must come onto the board', and 'It cannot move within 1" of an enemy unit'.
2. The outflanking unit is delayed until it becomes legal to come onto the board. If the game ends while the unit is stuck off of the board, it gets destroyed as per the reserve rules.
3. Because both options 1 & 2 are boring, you should fabricate some house rules, such as allowing the outflankers to shoot and/or assault the blocking unit in an effort to get enough room.
Note that neither option 1 nor option 2 are spelled out by the rules, and there are precedents for either to be applied.
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