What is 'less' about 'unless'?

Solution 1:

You would do well to read OED more carefully. OED 1's definition for the earliest meaning is

  †1. prep.phr.  On a less or lower condition, requirement, footing, etc., than what is specified.
  With preceding negative, express or implied. [my emphasis]

That is, the sense is "not on a less compelling condition than" what follows, NOT "On a less compelling condition than" what follows.

In modern terminology, on less (like modern unless) is a Negative Polarity Item: it only occurs in a negative context.

Here's the first quotation OED 1 gives, in modern spelling.

c 1400 [...] But that may not be upon less than we may fall toward heaven.

That is, "That cannot happen on any less condition than our being able to fall upwards."

1475 [...] His land, which many people .. fear to take .., unless than they might be made very sure of payment.

That is, people will not take the land on any less condition than that of being assured that they will be paid. Here fear supplies the necessary negative context.