Well, your formula is just not right in this place. We don't want to find out when it becomes more likely for a foreign merchant to beat yours, that doesn't matter at all in this consideration. We just want to find out at which mean value your merchant with 99% compete chance (against EVERYONE) will lose against another merchant. And that is as simple as it looks: 99% chance => statistically the 100th foreign merchant will compete you out.
No. Statistically one of every 100, not #s 100, 200, 300, etc... And the simple fact is your coastal, high value, CoT is pretty likely to get hit with 100 traders a year or more once the province is known to the latin tech group. The a.i. traders focus on high value CoTs as having the greatest potential return. That 1% chance can turn their 2-3 ducat cost into 8+/month in a CoT worth 1800. Since it can easily take you 2-3 months to get merchants there to kick them back out one success can easily pay for 5+ failures.
If you want to dominate a CoT the best way is to put it somewhere out of trade range and hidden from Europe. The best locations are inland as coastal provinces are too easily reachable.
As a free trader you can completely monopolize a number of CoTs, but to do so you will have to limit the number of CoTs you trade in as well as other maluses like uncored CoTs. I have been able to maintain up to 20 13 empty chair monopolies by only trading in those CoTs and predominantly trading in distant overseas markets most of Europe couldn't reach and/or didn't know about (slow map spread ftw). Having a massive trade tech lead and eliminating most of the free traders helps too.
While the ability of free traders to dominate more CoTs is a problem, you have to consider that in general it requires a much greater investment to do so. Most nations start heavily mercantalist and full blown free trading pretty much requires SCP as well as NTP. You also lose some pretty nice trade income modifiers/decisions. IMO the compete chance malus for number of placed merchants needs to somehow consider open seats in your monopolies and it should also be an escalating, not flat, penalty. It is somewhat infuriating to be unable to maintain more than a couple full monopolies of 100% owned CoTs as a full merc nation because of the malus for placed merchants when you are only trading in owned CoTs. Perhaps removing that penalty in owned CoTs?