Certamen as an Ability just moves the Tremere's extra cost from their Virtues to their Abilities. This matters less now because the Mystery Houses have similar Ability point pits in their cult lores, but basically it means if you are an adult Tremere you need to spend 75 or so XP on something without a lot of game effect.
Certamen only works if the Order is an Honour (Shame) culture where refusal matters. After the Sundering it becomes a Fear culture or Guilt culture, I'd argue, which is easier for real world players to understand.
The way around this, IMO, is to attach punative Reputations to those who refuse to duel. Is that fun though?
On the other side, even by its own lghts, a Certamen Focus is, admittedly, a problem because it undermines dueling as a dispute resolution mechanism. One of the core tenants of most duelling cultures is the ancient Irish idea that the blades must be of equal length. A certamen focus means Tremere are always bringing a short sword to a dagger fight.
This changes the historical calculation. In, say, Iceland where you took turns trying to smash through the other duelist's own personal shield with you own personal axe, you might not want to duel the guy who was eight feet high and had an axe twice the weight of yours. Better to take you case to the lawyers.