Of course, if the respondent can choose a champion, then the challenger as well can. If his cause is just, he shouldn't have a problem - if.
There is a bit of an "ole!" factor in the respondent dodging out that leaves a bad taste, and it can turn the problem around- a passive-aggressive mage who walks around with no great fear of certamen, knowing that he has a "big brother" who is (usually) willing (for whatever reason) to fight his fights for him.
However, the idea of a duel, of whatever sort, being fought by proxies is one that worked in RL in several cultures and several different historical periods, so... why not here as well? History knows more about it than we do, truth to tell.
Right, I agree with everything you've written and should note: I did not suggest that the use of proxies (champion(s)) has no place, it certainly is in line with the setting (so long as that is a condition acknowledged openly by one or both parties prior to final agreement of terms and conditions and awards).
That said, in all perhaps but a few settings I do not see this practice being widely adopted as an OoH rule (as a matter of personal opinion). I perceive the general attitude within the Order as being one of self presumed superiority to the conventions of mundane society (even that of nobles) by reason of the power possessed by even the most average magus. Thus "what petty little mundanes may find acceptable practice" as I perceive the popular OoH thoughtline to run, "is beneath any magus to emulate".
Exception to this might be the Jerbiton who are so fawningly attached to mundane fashion and practices.
Everyone else's perception of the prevailing OoH mindset (in this regard) may of course differ from mine.
I agree that proxies/champions are reasonable, though in most cases it should be part of the negotiation of terms. Though Paris Sophia's example of a master magus having a filius duel in his place is a reasonable exception. It's the equivalent of the master saying "Your challenge is valid but you're not in my league, so here, duel my student." Against a fellow Master, this would be a serious insult but so long as he was willing to abide by the terms should his proxy lose, I can't see a Tribunal objecting to the practice.
Proxies who are obviously stronger/more powerful would most likely only be acceptable if previously agreed. The exception might be the Tribunal stepping in to deal with a bully.