She looks as if she is farm / working bred. The smooth coated ones are much more popular on farms where their coats don't get laden down with mud and muck after a day on the farm. Most pet and show ones are bred with longer coats, plus because they're not being bred for working a laid back temperament is more valued when they're being bred as family pets and temperament is inherited to some extent.
Mine are a halfway house really, bred from working lines as I only sell them to people who do some sort of work with them, but because they will all live in houses temperament is a factor too so they can't be the very sharp ones that farmers might breed from. That said I have one who is a bit of a guarder and who would probably bite someone if they broke into the house or garden. Not enough to do serious damage I think but enough that they'd know about it. She's ok if we're here and people visit tho as our acceptance of that person gives her the message that they're ok but the guarding instinct is definitely in her make up.
Collies can be quite noise sensitive (I have a couple like that) and you'll often find that the younger dogs take their cues from the older ones. So if your old dog was non reactive in this sort of situation she will have picked up on that and taken her lead from him. Now that he's no longer there her stable influence has gone, and her own natural temperament is starting to come to the fore, and her way of dealing with things is very different from his. Plus you're all she has left as she probably had a good bond with him too. That's what it sounds like to me anyway. She will have gone through a period of adjustment when he died and then the behaviour started to manifest after that. I'm guessing he went 6 - 8 months ago perhaps?
I'm guessing that you are the one she bonds with so anyone else is seen to be a potential threat to the attention you give her. So it sounds like jealousy guarding with a bit of territorial guarding thrown in for good measure as per her being uncomfortable with people wandering round the house. Add to that the fact that she's also nervous so likely to be more reactive to different situations and you get the dog you have.
To some extent this is something that is 'in' the working breed but I wouldn't just accept that this is the case. Take her many places to ensure she gets plenty of exposure to different situations and learns to deal with them, and if you can have her in other people's houses with you sitting on the sofa and see if the same thing happens or whether it's just connected to your / her own house. If it is just your house (my guarder would only guard her own house, never anyone elses) then you know that it's the location she's guarding as opposed to just you. If it happens elsewhere then it's you she's guarding. If you can work that out it helps as knowledge is power, and understanding why she does it is important so that you can then devise a plan to help.
I've never used them but some people use herbal remedies to assist with this. So something like this:
Calm Down! - CSJ Herbs for Dogs - CSJK9 Ltd.
Or some people use a few drops of Bach Rescue Remedy and swear by it, but as I said I've never used anything like that so can't recommend it per se.
I'm no behavourist by the way, I just have many years experience of dealing with this breed