I mean, they wouldn't GUARD the house if there was nothing there.
You are assuming that it wasn't the
authorities (to use Lily's word) that goes after MC's father...
Now, just imagine that it was them. Then, guarding the house is something mandatory. In case the father come back, in case the son come back, as well as in case someone wander in the house and find a proof they forgotten there.
And it happen that the
authorities, or at least a group among it, is the most logical suspect.
MC's father was one of the Seven, and Lily told us that it mean he was the most powerful magic user of his world. Whatever the magic potential of humans in regard of other worlds, it's clearly not someone that you would attack alone. And like he's one of the Seven, it's also not someone that you would attack without "political support".
The fact that he was one of the Seven also imply that whatever he did that pissed people off, he was alone on it. I mean, when it was decided to that "killing your food" was now illegal, it pissed of demons and vampires. But it was a decision from a group, killing one person, even if it was the one who convinced the others, would have changed nothing. This while killing MC's father clearly changed something. They missed, but he disappeared and was supposed dead from any magic user; whatever he wanted to do ended with this disappearance.
So, you've the most powerful magic user of a world, one of the Seven, that is attacked in his house, assumed killed with his wife and son, and 20 years later the
authorities still haven't found who killed him. How believable is it? How the 6 others, and I assume his successor, haven't forced the
authorities to use all the possible means to investigate, by fear to be the next one? Or, more precisely, how is it possible without complicity inside the
authorities, and possibly also among the Seven...
And jerk MC's dad faked his death a second time, sending his son in the viper nest, only providing him a trustworthy guide, Lily.