Re: the RAW - my issue is with the description of the second case. To me, it sounds like the first case is describing the magus creating the circle (with the concentration check). The second is describing the magus simply tracing the circle (without a concentration check.)
Because if you need a concentration check for both, the rules are identical - and thus there's no reason to split out the two possibilities. Alternately, the author would say "If the circle already exists, the magus can trace out the circle. However, all other rules, including the need for the Concentration effect, still apply."
But it doesn't say that - it just says you need to trace it out. Admittedly, the rules COULD be simply confirming that you don't need to physically gouge a ring into your stone floor in order to cast a Ring effect: the permanent inlaid ring is enough. (and I certainly agree.) But the fact that it doesn't mention the Concentration roll, to me, makes it unfortunately unclear.
Re: the Concentration roll itself - I actually don't have too much of a problem with that, as the scenario I'm describing is not stressful - that is, it's being done well away from the actual location of the covenent, no one is trying to shoot the magus with an arrow, etc. - As casting magic is not, by definition, a stressful situation, it falls under the standard rules for defining a stress roll. Which means as long as your magi has an Int+Concentation of at least 6, there's not going to be a failure.
And if the GM is going to say "no, something that large is going to be stressful" - well, OK. If that was the case, then the magi would know how large something would have to be before they started rolling for stress - which your solution (using the Concentration rules) would likely indicate: 15 minutes/level of Concentration.
Which, actually, is pretty big. 10 paces/turn = 5 feet/second = 4500 feet per level of concentration. Which is probably more than enough to cover a whole covanent. A magus with Concentration(spellcasting) is going to be able to cover a 9k circumference circle, which has a radius of ~1400 feet. Again, that's pretty darned big.
Re: breaking the ring - sure, that seems to be an issue. But in this particular scenario, the ring itself is being prepared well away from the covenent, using a ReVi delay effect. If the demon were to come out and try to break the ring, it will have to both know that it is being done, and will have to leave the covanent to do so. But getting the demon out of the covanent is the point of the effect anyway, so that's actually a success.