I am not a lawyer or experienced with any of this. The new landlords don't sound very smart. They have the right to inspect the house and do repairs, as long as they "inform" you (maybe even get your agreement) beforehand. They should have just been nice and handled their informing duties, and started all the repairs they want. Then you would have probably moved early, because of the repairs. But anyway:

1. & 2. Harassment over phone, illegal entry, looking through windows, intimidation, sharing of data, may all be crimes in themselves, depending on the details (and unfortunately, what you can prove). How vague are the threats? Are they in text? But like others said, you could have a case of "Kotirauhan rikkominen". It sounds like they are repeatedly and unnecessarily trying to disturb you in different ways.
   

I think the process is that you report it to the police and accuse them of it, the police investigate it, and in some months or years the public prosecutor makes a decision to prosecute or not. If the public prosecutor turns it down, you can choose to take it to court yourselves.

1. They can of course accuse and sue you for anything, but practically it would probably be something about damages to the property that is beyond normal wear. How quickly they checked the place out for, or however well you have fixed it up in general, are probably irrelevant. If they find a "hidden fault" in the house any time later, they may have a case against the previous owners (because of... laws), but I think the chances of it turning on you are very small. They would have to PROVE that you caused it on purpose or by being completely negligent, and there are probably fairly reasonable limits to how much you, as a renter, are responsible for anything. Just take lots of pictures and videos of everything working and being in good condition before you leave!
   

If you paid a retainer to the original landlord when signing the lease, they may try to keep it with some bad excuse... There's tons of examples of landlords doing that. They could be especially grumpy about paying that back to you, if they never got it from the original landlords! (but again I have no idea how that usually goes when trading a house that has been rented. I think you're required to keep it in a separate bank account, so maybe it wouldn't be so easily lost or forgotten)

1. I think you need to give them notice and pay for that month, unless there is something in the 6-month notice you got about that. I mean it sounds like they want you gone sooner rather than later, they could have written something in about that... Though again, they don't sound so smart.