Coin-holding-time-weighted voting mechanisms are a bad design that is far too easy to manipulate. By making different coins have different amounts of voting weight, you’re creating the incentive for relatively simple types of collusion attack in which parties collude to make it appear that coins are not being spent, while actually spending them amongst each other.
Since there’s no technical mechanism that can completely prevent this form of attack, you’re basically stuck with it — and it becomes inevitable in the event that a specific election matters enough. On top of that you’re creating a whole batch of security issues and perverse incentives, as you mention in your post.
If we must do coin-stake-weighted voting, all coins should count equally. This isn’t perfect either, but eliminates a huge class of opportunities to game the system.