I've searched and found nothing, so hopefully I didn't just do it wrong somehow. This is kinda two topics in one.

First of all, shortly after acquiring Battlehorn Castle, the maid Rona took to following me into Chorrol every time I went to sell some of the wines I made (I was RPing as a vintner at the time). She would show up, then turn around and walk back home. As punishment for leaving the castle, I started locking her in the hidden room in my quarters every time I stopped by, followed by the occasional attempt to kill her. Clearly, she didn't appreciate that. She started showing up on my heels everywhere I went, usually whenever I fast traveled somewhere. Once again, she'd show herself, greet me like I'm some kind of scum, then begin the long walk back to Battlehorn. And she would indeed walk the entire way. For several weeks of real time, it was not uncommon for the words "Rona Benanius is unconscious" to pop up on screen as she came across some creature that disliked her nearly as much as I do. It was amusing for a while, but eventually, I got sick of seeing her every time I didn't feel like walking to my destinations. So I decided to head to the Shivering Isles, figuring she wouldn't follow me there. Wrong. The first time I tried to return to Cyrodiil to drop off some valuables, Rona was right there by the table where you meet Haskill, just standing there. I tried to talk to her, but instead of being her usual annoying self, she simply said "I HAVE NO GREETING." That's probably because she's not supposed to be in the Shivering Isles. The game doesn't know what to do with her anymore, so her AI is offline. She's been there ever since. Now, I have no problem with the pilfering maid out of my room, but this raises a few obvious questions.

What is she doing there in the first place? Why was she following me for almost three months? Has this happened to anyone else?

And as for the guard... When I first started this character, I decided that I wanted her to wear a Chorrol guard's armor, but didn't want to deal with the bounty. So I set out some poisoned apples in the barracks and waited for someone to take a bite. My first victim died as planned, I looted his corpse, removed all my apples, and went about my merry way. Everything was fine. When I returned to Chorrol several days later, I notice a guard's corpse under the great oak. On my next visit, there's a dead guard by the gate to the castle. The third time, I actually saw it happen. The guard's health bar was visible and dropping rapidly, until he collapsed in the middle of a conversation about Eugal Belette. Clearly, the poisoned apple was not satisfied with simply killing him once. Its effect has stuck to him, so that every time he respawns, he soon dies. He doesn't like me very much, by the way. His disposition toward me is at a round 0. So again, has anyone else seen something like this? Surely I'm not the only one who poisons guard for their uniforms.

This is on the Xbox version, by the way.

From the UESP;

I had not asked her to follow me, but I just found her in Memorial Cave. She had none of her usual dialog options, only the generic ones, and she didn't sound like she liked me much.

I found her in oblivion and her only dialog option was Rumors. every now and again she appears, in the imperial city, in a dungeon, in the wilderness and I have no way of dismissing her

She sometimes shows up randomly when you fast travel, and briefly talks to you (non-dialogue) before heading straight back to the castle. Going back to the castle, asking her to follow me, and then asking her to return to her duties fixed this for me.

I realise this conversation is mostly old, but I just found Miss Benanius in the middle of the Oblivion gate on the peninsula northeast of Leyawiin. In my file, she's never gone anywhere except the private quarters of Battlehorn Castle, so this was quite a shock to me. She was fleeing; I killed the Dremora and she started talking to me like normal. Don't know if this is important or not, but there ya go.

I just found her at Sideways Cave. After clearing out the cave, on my way out, I found Rona on her way in. Her only dialog option was "Rumors." Now, at one point inside Battlehorn Castle I asked her to follow me, but then dismissed her before even moving. I never noticed her following me. After speaking with her, she left in the general direction of Battlehorn Castle, though I didn't follow her, and it's a long way.

I don't know if that helps, but a t least you're not the only one. As for the guards, NPCs that respawn (like guards and bandits) will pass this effect on to their next respawning copy. This will result in these people dropping dead for no apparent reason. This is because the script effect does not tell the game to remove the ability after the actor is dead.

Sorry I can't help you more with the follower problem

I don't know if this will work just a guess, maybe if you delete battlehorn she'll go away too? Not sure if she exists without having the dlc. I have no idea if doing that kinda thing could corrupt your save so maybe get a second opinion on that one

Thats weird..I've had a Conjuror break into my secret room in BH castle..IDK how he got in there unless he can jump 100 feet in the air.