Do any social welfare functionals, other than maximin, meet all of Arrow's conditions plus invariance regarding ordinal level comparability?
There are at least two other examples of SWFs that satisfy these conditions.

The first is a positional dictatorship.  Let N be the number of individuals (assume it is fixed).  For any k between 1 and N, the kth positional dictatorship SWF orders social alternatives in terms of the preferences of the "kth best off" agent.  Formally, given any social alternative x, let v(x), be the vector of utilities of all individuals for x, but ordered from lowest to highest.  The  kth positional dictatorship SWF is then defined by the kth component of the function v.  If k=1, we get the maximin.  If k=N, then we get the "maximax" ---what you call the "inverse" of the maximin.  If k=[N/2], we get effectively "dictatorship of the median individual".   The point is not that these rules are normatively attractive (they aren't) ---but they satisfy your axioms.

Another possibility is the so-called leximin or lexicographical maximin rule.  This is the lexicographical extension of the maximin, obtained by ranking social alternatives according to the vector-valued function v from the previous paragraph, but with coordinates treated lexicographically.  Thus, alternative x is better than alternative y if it has a higher minimum utility value. If x and y yield the same minimum utility, then we compare them by looking at the utilities of the second-worst off individual in x and y.  If these individuals also have the same utility, then we look at the third-worst off individuals, and so on.

This SWF is very similar to maximin, but it satisfies a stronger version of the Pareto axiom.

For more information, I suggest you look at the 2002 article by Claude d'Aspremont and Louis Gevers entitled "Social welfare functionals and interpersonal comparability", which is Chapter 10 of the Handbook of Social Choice and Welfare volume I (Arrow, Sen and Suzumura, eds.).  You could also look at Chapter 2 of the book Axioms of Cooperative Decision-Making, by Hervé Moulin (1988).  In particular, Theorem 2.4 on page 40 of Moulin's book might be useful to you:  it says (roughly) that the positional dictatorships and their extensions (such as leximin) are the only SWFs satisfying ordinal level comparability and a few other mild conditions.