Anything that was implemented by theymos would either have it's flaws or be detrimental to it's users. It's no different with the current solution that's been implemented but, really I can't think of another way which would of been better and I'm sure if you can make any improvements on the current system theymos would appreciate a private message about the matter.

The way merit was introduced to a forum which had already been running for many years is going to have a dilemma on it's hand.  Either you reset everyones rank or you allow people to keep their rank. Obviously, the first option isn't ideal and a lot of people would be disappointed having to start again especially when there are limits apposed to those that are a lower rank which could mean that members who have been here from the beginning would have to wait 360 seconds just to post which could be detrimental to their usage of the forum. All of these users have been through this already and probably wouldn't appreciate it a second/third time around as I recall the way you rank up as changed twice before this implementation. For example, when I joined first of all it only required making a certain amount of posts and activity didn't exist.  

People who abuse the merit system heavily will be easily found out. If they keep sending it to the same accounts it becomes somewhat fishy and might be monitored by the default trust. It's up to the discretion of the person whether they feel a negative or neutral or any feedback is beneficial.

There was another approach that could of been taken and was allowing everyone to retain their current ranks but, not give them any sMerit at all and only allow merit sources. Although, in my opinion this would mean the merit system would of been harder to judge in the early days and would of taken a lot longer to take off. At the end of the day people making poor quality posts will not be able to earn any more merit and will eventually run out of it and not be able to distribute this to their alt accounts. Plus allowing members to send sMerit at the beginning was likely monitored by theymos to find suitable merit sources.

To your second question though;   There's likely no easy way to prove that you haven't been abusing the merit system. But, it doesn't matter too much if it's being abused on a small scale. It will likely only become a problem if a merit source was abusing the system and there's ways to tell whether they are abusing it.

I honestly thought we would see more untrustworthy people sending merits to those they dislike. I thought the likes of Lauda and is alts /s would be targeted by this just to make it look from an outside view that they were abusing the system.