Certamen champions are certainly allowed: it's the only way unGifted Redcaps and Ex Misc hedgies can even defend themselves under the code, in the context of certamen.
Yes, we allow it for those members of the Order (although the allowance for members of House ex Miscellanea without training in the Hermetic Arts was hard-won at Tribunal), but is it permissable for the bulk of standardly trained members?
And as Tremere willingly suggest that by going to their Parens for protection occasionally, they'd merely consider it good sense if another magus did hire a champion.
IMS certamen is only used sparsely. We do not like the mechanics OR the idea of certamen, since it basically means that you are a bully instead of an intelligent human being. Debates are way more common to solve disputes. After the Sundering we consider that Certamen was disgraced as a sensible meaning of solving disputes and is only used in that sense inside house tremere. For the rest of the orter it is there as a fencing sport, not a legal system. It ias ocasionally used, but not commonly.