You could go through the same amount of effort to forge a screenshot of a PM, yet screenshots are banned. Most people will not go through such effort to forge a PM, so if we see a screenshot, we know it probably is real. Same with the numbers: no one is going to do what you wrote to forge a PM. Just not allowing the conversation number in the PM quote would make it easier.
As pointed out in the post above all this is moot, since each reply in a conversation gets its own number and thereby doesn't give any hint at when the conversation was started.
But my method wasn't hard; the simply one at least. Let me give an example.
Your role PM is number 256421.
You want to fake a PM and make a new conversation; it has number 263871.
13 days has passed since you got your role PM and the game started; you claim that the fake conversation started on day 6.
The change in conversation number between your role PM and the conversation you started when you write the fake PM is 256421-263871 = 7450.
13 days has passed giving 7450/13 ~ 573 new conversations a day during this period. You claim the fake conversation was started on day 6; well then it would have a number in the range
7450/13*6+256421 = 259859 and 7450/13*7+256421 = 260433.
If you claim the fake conversation was started shortly after deadline day 5 (thereby early day 6) choose an early number in the interval; if you claim it was started shortly before deadline day 6 (i.e. late day 6) choose a late number in the interval.
It is basic math and it would be rather hard to detect unless you are so unlucky to hit a number somebody in the game has.
But as mentioned it is moot since each reply has its own number; just wanted to show how simple my original idea was.
Lets test what happens if you by chance hits a conversation message number somebody in the game has used. I have taken the conversation number of a reply LK made in my pack conversation, but I have changed the name and member number to RRW. Lets see if I, MC, EURO and LK are still taken to the new standing list LK made.
Edit: I am still taken to the correct conversation message. So if you are so unlucky to randomly hit a number which refers to a reply in a conversation somebody in the game is a part of, your fakery will be foiled. On the other hand what is the chance of that happening.