Is there an English translation of Minding's 1839 paper?
Is there a translation of Minding's paper?" Well, yes, there is one from 1956 in the book Об основаниях геометрии. Сборник классических работ по геометрии Лобачевского и развитию её идей. You can download it here, Minding's paper is on pages 166-167.
Is there a simpler or more recent proof of the same theorem? The theorem is given as an exercise with solution (number 1.2) in this differential geometry course by Michael Eichmair at the ETH.
You can also find a proof on page 292 of A New Approach to Differential Geometry using Clifford's Geometric Algebra by John Snygg, and on page 282 of Differential Geometry by Erwin Kreyszig.