We are currently running a saga in Normandy in which the tribunal's tournament has its importance.
During the player's first tribunal, the tournament took a whole session (I am talking 12-15hrs) counting, inevitably, some time to discuss house rules...
We played each challenge in detail, including the melee with grogs.
I have not stated all the opponents. In my experience, the most complex is the Hastilidium. With 3 magi per side, with each team having their own strategy, it makes for a lot of spellcasting.
For the certamens, I base the opponents on the Arts of the player's magi, factoring age, and give approximate values. The players have Order of Hermes Lore rolls to see if they know the reputations of the opponents, their strengths, their weaknesses.
So, I would say that I "wing it". Of course, with time, it builds the population of the tribunal (and surrounding). I included a champion for the Dimicatio, an old Flambeau magi, who starts with a BoAF with increased magnitudes for fancy effects. If the opponent dispells it, then he takes the ca. He like to think of him as a (medieval equivalent of a) star. The best tip I can think of is to back opponent's strengths and weaknesses with interesting personalities.
One effect the tournament had is that, between the player characters, Dimicatio is often used to settle disagreements rather than Certamens, which is something that came from them. I think this is particularly interesting : it shows how young magi react and adapt to the environment. So, naturally, this and the rewards from the tribunal made such that early in their careers, they did focus on trainings for the tournament, but I don't think it would be necessary. It is true that they started young magi in an Autumn alliance, where they received their 1 mandatory vis per year, and had to work hard for any extra ressources. I think it contributed to driving them to train for the tournaments.
We did not play much with the plotting potential of the ranking, so I cannot comment on that one.
We did 4 tribunal until now, I think. The tournaments were not played in such detail during the last two, but we keep track of the team's successes and failures.
So, that is how we play it. Hope my comments help!