1. He cannot force you to sign anything in addition to the existing lease, and cannot just arbitrarily make up penalties for refusal to sign something you aren't required to sign.
2. He does NOT need the waiver to show the house. He has that right automatically under standard landlord-tenant laws. It doesn't have to be written into your lease. So you cannot stop him from showing the house.
3. HOWEVER - you are still entitled to 24 hours notice for each showing just as you are entitled to 24 hours notice for a maintenance visit or any other non-emergency entry.
Therefore your landlord has the right to show the house, he just has to state in his listing that all showing require an appointment with at least 24 hours notice and he must relay those dates and times to you by posting on your door or some other mutually agreed method such as email or text message. You *could* waive the right to 24 hour notice but you are not required to, and since you don't want to, I don't think you should.
3. The statement about not being liable for theft is meaningless, because they already aren't liable for theft caused by others. If someone comes in as a realtor or potential buyer and steals something, then that's their crime, your landlord or his listing agent aren't responsible for crimes committed by other people. The statement is just there to hopefully prevent you from suing them. You'd lose that lawsuit anyway, but they're trying to save themselves the hassle of defending themselves against the frivolous lawsuit because not getting sued is still better than getting sued and winning.
4. Unfortunately you are right to be concerned, theft does happen when an occupied house is being shown for purchase. You should take action to protect yourself - most importantly lock up all valuables and hide them or even move them to off-site storage or a trusted friend's house. Also make sure you have a renter's insurance policy that covers theft. If you don't have one then go get one today (its really cheap, like $15 per month). If you are really worried, then installing surveillance cameras would not be unreasonable but it doesn't really help other than maybe providing evidence for a lawsuit or criminal case against the thief.
I think you should refuse to sign the waiver. Tell your landlord you will allow showings with the proper 24 hour notice as legally required. You refuse to waive your right to the notice period and refuse to sign any waiver limiting your right to sue for wrongdoing. If he attempts to fine you or charge you in any way (including invalid withholding of the deposit), then you will take legal action against him for refusing to honor your rights as a tenant for the final months of your tenancy.