I would expect that an OPC would have the same ability to reset a squatter's rights timer, as an NPC or PC. Since to the game's mind, they are all equal. So I wouldn't worry too much about a squatter coming in and sitting in your home or shop for a whole month and laying claim to it. Since your OPC would no doubt return home to eat and sleep, and also return to whatever business they run. So any land you own should get reset automatically every time your OPC went through it's daily routines.
As to people realizing that your character is an OPC and not actually you. I imagine that it would be fairly easy to tell if they simply watch the OPC for a few days. Since it will undoubtedly walk the exact same path and do relatively the same thing every day. They could tell due to the lack of variation. Leaving you open to any form of plot they devise. Such as stealing from your shop while your OPC is at home, knowing that it won't be back for a set amount of time.
Though, if SBS makes an app that lets you swap out OPC scripts on the fly, I would expect that the randomness caused by that, would throw most crooks.
Your problem is quite the quandary. I hope that SBS has thought of something to minimize the difficulties a situation like this could create for people like you, who's jobs and daily lives interrupt any free time.