I don't agree this is a case of squatters. IMO, that implies people who set up residence in the home when it has been abandoned.
If someone is murdered, or disposed of by people who want to live in the home, they would be 'murderers' or 'fraudsters'. They would be committing major crimes of theft of the value of the home, its assets, and the benefits; identity theft, and possibly forgery, illegally disposing of remains, etc, and would receive jail terms because of the level of greed and malice that would motivate such crimes. There is no defense to crimes of theft and fraud called 'squatters rights'.
I believe it takes a lot of time to build the case, because of how far back it goes: LE will try to determine, as precisely as they can, when Mary Margaret died. Or at least, the last date she was seen alive.
From that point, they will look for evidence of actions taken to conceal her death, steal her benefits, etc: who precisely undertook those actions, precisely when, precisely how.
LE/prosecutors can't be vague, or charges won't stand up in court.
In one case, the perpetrators produced letters from various locations/letterhead, purporting to be from the victim who had gone travelling, which were proven to be fraudent because police found a hotel receipt proving the perps had been staying in the hotel at the time a letter was sent.
In another case, the perp had to get a woman to impersonate the victim in an interview with social services - eventually she confessed in exchange for immunity.
If pension/benefit companies are smart, they should periodically send a letter requiring the recipient to sign off that they are who the benefits are intended for. So the person collecting benefits has to commit fraud to continue to receive them. Otherwise, there is potentially an excuse of ignorance, mental health problems, etc.
JMO