No, that estimation includes the threats by long range attacks (like the attack you described - the typical "old keys attack"), bribe attacks, short-range attacks and other known N@S-related scenarios. No one of these attacks is free, most of them are highly impractical (try to find people that sell you 50% of the staking amount in some moment of time) and, thus, expensive

However, I believe a PoS attack to be in the same order of magnitude (~3-5% of market cap).

Sybil attacks can place a fake chain with a lower PoW difficulty rating until a node sees the other chain with the higher difficulty and reorgs.

You have to buy or steal the PoS coins to stake them?
Their is a cost involved.

Also your pretense at how easy it would be is over exaggerated.
Feel free to 51% attack zeitcoin to prove how easy it is for you.  
(According to you it is a zero cost attack so nothing is stopping you.)

What I am saying is if their is a sybil attack involved and your node is being blocked from seeing the true chain,
you can use a block explorer to verify the true chain for PoS or PoW.

True chain can be determined by comparing block height with the block explorer for PoS or PoW.
As a Sybil attack can fake a chain on either PoS or PoW and only comparing to a Block Explorer can verify the true chain for a syncing node.

It's a difficult attack to be sure because owning that much stake in a network is unlikely - but it is absolutely not impossible because many PoS systems especially have very lopsided distributions. Losing the ability for new nodes to know what is the "one, true chain" without needing outside information is a problem. How big of a problem is a matter for debate, but it can't just be brushed off as so unlikely as to be impossible.

I guess OP would say B should never witness both A2 and A2+, users are supposed to get enough witness for A chain before considering a transaction confirmed.

In Figure 6, user A attempts to publish alternate blocks A2† and A3†. Since user B3 has already wit-nessed blocks A2, A2† would be rejected by all participants. Furthermore, A can be flagged as a malicious or compromised user for presenting two different solutions for A1

What I am telling you is , you are wrong.

If you modify the wallet client to place false time date in the blocks , all you are doing is making a hard fork that the other nodes will ignore.

Cost is not the issue, Each Block has a defined target of say 1 minute between blocks.

Your chain is 3 months behind, and still has a target time of 1 minute,  your block height will always be ~ the same 3 months behind and as such never a threat to causing a reorg, because a reorg can only happen if your block height # exceeds the main chain.

So how do you make up the 3 months time difference?

FYI:
Any change to the code to modify the time target between blocks could allow faster blocks, would lower the target difficulty making it a weaker chain and also break consensus with the other nodes, therefore making sure it would never be accepted over the main chain.

FYI2:
The phrase (block production has zero cost) , is incorrect.
There actually is a cost , it is time.  
Your block has to wait the coded time before block generation can occur, and those coins go dormant for a coded period, another time factor.
The Time between blocks is hard coded which affects the difficulty # in proof of stake coins, thus defining the strength or weakness of a chain.

My question is this:
Let's say their are no checkpoints , rolling or coded.

Someone buy the old private keys or at some point just actually owned over 51% of a coin total.

Say they try your attempt , but it was 3 months earlier when they owned coins.

The Blockchain has 3 months of confirmations ahead of them at a rated say 1 minute interval.

How do they ever catch up , with the block height of the main chain, won't they always be ~3 months behind?

* Now if you say it is possible to trick the time setting and somehow condense those 3 months into a day, please provide details or proof on how that is done.*

This would be a perpetual subsidy - not one that runs out ?

Is there no competition to mine the most max POW transactions, and accrue the most mining subsidy ? A professional miner as you say in your paper.

.. there you go!..  

The Mining Subsidy.. It's not clear to me quite how that works..  What Stops people fighting over it  - or do they just compete as normal ?

How long does it take for a branch to become heavy enough to be considered final ? How do you remove the need for the coordinator that IOTA uses ?

Are you knocking this up and launching ? ..

A system like IOTA. The work is the fee. (But not IOTA - as it has it's own quirks. Something new.)

Why wouldn't the "community" be resistant to this as this poses a threat to the security of the network.

It's simpler just to interchange USD for stableUSD directly on a one-page website, without using any tech inbetween (coloured coins, whatever). The Federal Reserve accepting 1 stableUSD for 1 USD is the most guaranteed peg possible

Right, it's the same model as Tether, but with either the issuing central bank or Wells Fargo etc doing the 1:1 redemption

Either the coins are in the ACH legacy banking network, or they're in p2p stablecoin network. Neither permit double spends. It's simple.

No the idea would be to use a p2p network protocol to allow permissionless transfer of the stablecoin, but use direct convertibility with the issuer to maintain the peg.

Central banks would be unlikely to take that approach, but in principle there's no technical reason why not. The only reason I have for saying central bank cryptocurrency would solve the stablecoin issue is the simplicity, why use omni layer when they could just redeem TCP/IP routed USD 1:1 directly at federalreserve.com, or through whatever regular commercial bank?