FoE is an old game. Yes there have been exploits of getting diamonds by doing loop quests, and there have been other options. As far as i know all known exploits have been handled and shut down, so any manual to these abuses are outdated. Diamond farming is the only legit possible way, since it actually does not breach rules, this implies playing multiple worlds, and all secondary worlds are beefed up with wishing wells, since 1 well gives diamonds once per 100 days it is not that efficient, plus you have to actually GET wishing well to start this.
As to hacks, game developing standards have gone up over the years, so tweaking data transfer is almost impossible for two reasons: the information goes trough validation, and each change is recorded in log file. Although log files are not kept for long, significant changes in a single person can be noticed (just like with multi profiling, giving FPs to GBs).
Another type of hack is server hack. Ok, this one is VERY illegal, usually there is extremely high level of protection for the system, taking head on is near impossible, so better angle is going for human error, looking for a back door. Theoretically it`s possible, but in reality hackers of such level have more fun from programming and hacking, than gaming, so they won`t come for free diamonds. Worse case any server can have is a total wipe-out of all the data. Building code and system takes time, and even deleting one server could permanently cripple the game.
In theory, yes, any on-line digital system can be hacked, but modern standards are high, and if Inno were not up to this challenge, than we would not be discussing this today.