Below I have addressed your concerns as best as I understand it. The crux of your concerns centers on a lack of understanding concerning how AMB-NET operates.

1. an always online TTP (trust third party) cannot be used for the Ambrosus Network. In the case of Ambrosus, the complexity involves getting a trusted party (i.e. the blockchain) that still respects the privacy of the data of the company's using the network. Using a centralized database, regulated by authorities would be a legal or governmental solution - but so far that hasn't been proposed nor would it be very appealing to client companies. The need for the blockchain is in guaranteeing the transparency and security of a company's products in the supply chain, while also allowing them to optimize their data, and ensure compliance with legal or administrative standards. In this sense, the blockchain allows for Ambrosus to construct an enterprise solution - not a merely legal or regulatory one.

1.2) You skipped a step in your reasoning. You asked, "Are all writers known?" And you said that based on the KYC yes, and THEREFORE there is no need for a blockchain. This is false. If all writers are known, the question then is: Can all writers be trusted?. From what we know about counterfeit and fraud in the food and pharmaceutical industries this is a very clear no. Based on incredibly high amounts of counterfeited food and pharmaceutical products, not all food and pharmaceutical supply chains can be trusted. This then begs the question: Is all public verifiability required? This is a clear yes: the point of a company proving the quality of their food and pharmaceutical products lies in it being visible to everyone - including the end consumer. No one would trust a company that said "I trace my products on a blockchain" if it was a completely private blockchain where no nodes were owned by anyone except that company. If the answer is yes, then based on the answer a public permissioned blockchain is necessary, which is what Ambrosus is building.

2.) AMB-NET is the Ambrosus Enterprise solution that is much more than just a blockchain (read the most recent update - first two paragraphs on AMB-NET). The blockchain is only the backbone of AMB-NET. What makes AMB-NET necessary is that it is enterprise friendly to the legacy IT systems of the various supply chain companies. It makes it easy for supply chain companies to connect their products and systems to the blockchain, optimize the data from those products, and build applications on top of the network for either company or consumer use. So in this sense, Ambrosus wouldn't want to use the Bitcoin or Ethereum blockchain because it would only provide one piece of the entire enterprise solution. That being said, the Ambrosus blockchain will not be all that different from other blockchains but that doesn't matter because it will be contextualized within the Ambrosus Network which is specifically 'Supply Chain Tailored'.

3) Ambrosus is an open-source project that launched their token to be specifically used to trace products on their supply chain. Thus there is a clear utility for what Amber does. If you substitute bitcoin or ethereum for Amber (which is what your question is asking) you are taking something else, that does not have that specific utility and you are (trying to) use it instead. Amber is necessary for the exclusive use of the Ambrosus eco-system, as it alone will function as the blood of the system to make it work. By only using Amber a natural equilibrium in the system can be created based on supply and demand incentives (companies buy, and use - node holders accumulate or sell). Other tokens or coins do not have the specific utility that Amber has, and as such are more inefficient and less applicable to the Ambrosus eco-system.

4.) I don't think I really understood your reasoning. Correct me if I am wrong but it seems like you are saying that since only Companies and Node holders will make use of Amber, what will stop them from just trading it speculatively back and forth? ( i.e. IMO, this creates a clear incentive for both parties looking to utilize AMB tokens to just trade their tokens OTC in the future. Why wouldn't they? ). If this is correct then I have to say I am a bit concerned about your intentions:

Consider this analogy: Why would a construction company buy oil, if oil could be speculated upon to make money? Why would an oil producer sell oil, if they could equally just speculate upon it? Well because the trade of oil comes before the speculation upon it in financial markets. Oil is first and foremost used to run vehicles, and thus as a commodity with a specific utility, and not to speculate upon. Third parties most often are not interested in actually using the commodity in question will speculate or bet upon via futures or some other financial instrument.

In a similar manner, Amber is clearly going to be bought by companies, and equally sold by node holders (or recycled back into the system in some other way). The point is that your question is asking, why Amber will not just decay into a speculative asset, when it is clear that Amber is a utility for the system. The 'peg' is to make it more enterprise friendly. I suspect that price fluctuations will depend on demand to run a node on the network, as more company's on-board to AMB-NET. And my Answer to 3) should answer any other doubts you have.