Thats a moot point, if you own the majority of nodes in any system you can do whatever you like.
No it isn't. This is a critical difference between POW and all other designs; owning a majority of POW hashing power gives you the ability to attack the network, *but* you have to expend energy to actually do it, and that changes everything.
Therefore if you want to vote on the outcome of a conflict, you MUST of done work and been endorsed recently. Historic weight with this change is no longer a factor, so the long term con as you put it is gone.
What does it cost my majority of nodes, then?
Its no different here, except you have expend fees too to create the transactions as well as energy.
Cost depends on the volume of transactions the network is currently producing, and your act of producing spam transaction can elevate the cost further.
A simple breakdown using 100tx/s of network load as the numbers are easy:
The base fee at this load is $0.01, so the network is spending $1 per second on fees. You therefore also need to create an additional 100 tx/s to be certain that you have the majority of endorsement power, and endorse your own nodes while creating these transactions. You therefore have to spend $1 per second on fees, plus whatever energy requirements to create 100 tx/s.
You'll also need a shit ton of nodes to endorse, as no matter how much endorsement weight a node might have, that only gets you into the vote...once there you have 1 vote with the same weight as everyone else. Even if you have 50% of endorsing power and direct it at 1 node, everyone honest in the network is endorsing many nodes between them so at 100 tx/s of load, there will be quite a few nodes voting on outcomes.
All these nodes to endorse will cost to operate, and at 100tx/s you could perhaps get away with running 2-3 virtual nodes per 1 physical box at most without issue.
Sudden increases of transactional activity hike up the fee price, and is know as the load fee. Its unnatural and easily detected if transactions suddenly spike so your very act of producing spam transactions increases the cost to you considerably while the cost to everyone making regular transactions in a few cents at most. An extra 100 tx/s would bring the load fee to $0.04 making the cost of each transaction $0.05.
You'd have to sustain this load for 30 minutes for the endorsement weights to be mature enough to get in on the action, which would by that point have cost you $9,000+ in fees at this load.
If you do it over the long term and do it steady, then while you might bypass the load fee activating, you're having to pay long term to dupe the network into thinking the load it is seeing is normal....and your historic endorsement weight has no bearing on voting in conflicts anymore, as I've said, a few times.
At lower network loads the base fee is higher, so that the cost to attack it in fees is roughly the same no matter the load.