I'm going to use the alias of
You have already given here quite some details, hence somebody searching deep enough could get to you. It's a risk...
Then... well, I've seen places where the neighbors try to steal here and there a little bit of land by moving the fence by an inch now and then, so I can guess the stress, and the only solution I've found for that was concrete walls. Ugly, but effective. I'd also recommend, if it's possible and the law allows it, powerful security cameras, to also "see" a bit of what happens "around", just in case.
About threatening that they'll kill the cat... I don't expect the police be able to do much, it's the word of one person against the word of another person. The culprit will go free in no time.
Back to the blog idea, I'm not convinced any policeman would take it too serious and, in case the culprit is getting tipped off by the police (it can happen!), they'll know that you know this or that and when things get ugly, you'll become a target
I don't know, there has to exist a better solution...
RE moving boundaries, land limits are registered. Moving the "mark stone" is referenced in Egyptian scripts 2000 BC. and is still an offence so recurring to the law should work for that. Not that wall are a bad idea anyway. Catwise... well, CCTV and some recordings and recordings of sound could be of help, but...yeah... lost cause mostly.
RE old people living in expensive houses but with moderate pensions... well, you may consider a reverse mortgage or some form of equity release. It is of little use to have a valuable property if you cannot maintain it. You can live on it, get some additional income and still leave something for your heirs if that means anything to you.
EDITED TO ADD: I skipped over the fact that you know a few widows over 80 with valuable houses. Are they good looking? Would you say that they are in good health? .... just asking you know, they may need some love and sincere caring from me.