Starting with the most useful response first...
1) your excess construction costs are nil for a leased line. Only true if you are effectively on-net and there is sufficient capacity near to you from one of the large providers and/or where they are willing to write this off over a three year deal. Outside of major urban metro areas the ECCs for leased circuits can be hefty.
ECCs are no different between FTTP and leased lines, so the point is moot.
2) Whomever your agreeing to purchase that leased line understands that it is going to effectively be used as a sublet circuit for one to effectively act as an ISP. Could be especially problematic with Openreach tails.
Not really. You can run what you want on any leased line. Most people run an overlay network either by MPLS or SD-WAN, and you can do whatever the heck you want on it. Even on broadband. And leased lines are often 'sublet' even in office buildings, particularly when offices are sublet. How do you think the likes of WeWork works?
3) Actual monthly charges of leased lines vary massively depending on geography/location and capacity. getting a good deal in central London is easier than getting one in rural Suffolk for example, as I can attest.
Of course, but then again I'm based in London and leaning much more to metronets. But I know of several private networks on large estates and trust-owned properties that have a private network and a set of leased lines providing access to all properties on the estate, even in the darkest outreaches of rural Britain.
But the same model could easily apply to terraced houses, bigger blocks and such like.
4) The costs and ease to reticulate some form of connection to your neighbours etc are not insubstantial and there are ongoing run costs which are not zero. This is usually the deal breaker with these sorts of notions.
Of course not, but this is easily amortised over even the 3-5 year leased line deal. But in fact you'd be using either Mikrotik or Ubiquiti Edgemax gear in this scenario realistically which is not at all expensive. And there are private network companies that would install and manage the whole affair for you.
I know this, because I did exactly that:
easynetworks lit up 2x 1gbps Openreach EADs from fibre that was already installed to the main building on an estate in rural West Sussex (one using BTW and the other TTB, via separate paths), then hooked both up to 2 EdgeMax routers (£155 each) linked to a Ubiquiti OLT (£786) and an and trenched their own PONs to all the properties with each property having a Ubiquiti ONU (£35.11 each). Each of the 22 properties in this case pays £35/mth to easynetworks and everything is managed. Trenching costs of the PON and all equipment costs are all amortised in the contract. The contract for each property is 5 years of course, but no one argued as it was either this or 1-2mbps ADSL.
In fact, most properties were so happy they independently got easynetworks to further install and manage a UniFi system in their homes as well.
There is no "subsidy" on leased line installations - they are entirely commercial propositions. You pay a hefty fee for the first 3 years service, which normally covers the build cost (more expensive builds will have ECCs). Beyond that, you continue to pay a high monthly fee and there's a lot of commercial cream to be made from that. It's the razor blade model if you like.
...but ECCs are the same as FTTP on Demand or a CFP.
1. Of your cluster of (say) 12 properties, the users won't get *any* SLA: if they have any contract at all, it will be with their neighbour who buys the leased line. Nor will they get any consumer rights, Ofcom rights etc.
Says who? Surely that's an agreement between the company running the line and the other homes?
And anyone technical enough to run a leased line and associated equipment can quite easily provide support for it. But in such cases, it'd normally be outsourced to the company that lays the private network, such as easynetworks I quoted above.
2. They almost certainly won't be able to run fibre directly to their properties, hence it would have to be shared using wifi or similar.
Why not? If it's an estate it'd probably be managed by a trust. If a cul-de-sac it'd be owned either by a single freeholder or commonhold so they can do what they want. And if it's adopted by the council, it's actually much easier than you think to get permission from the council to run your own cables in a public carriageway - after all, councils tend to respect their residents' wishes.
3. It's unlikely that 12 adjacent properties will all agree to take the service, so it will have to be shared over a wider area.
It depends on the situation. There are many, many situations where properties are just leaseholders or tenants on a larger property, and this is purely a freehold or commonhold play.
However, in the context of a Community Fibre Partnership where they'd have to do that anyway, the issue is moot. I know most would much prefer to use a private company that works for them than use Openturd to herd such cats.
4. The person with the leased line has the contract with the ISP and is bound to pay for the line, but has to collect money from the individual users. They'll probably need to set up a limited company to cover their liability, and insurance if they are doing any installation work on the neighbouring properties.
In such situations it'd usually be a freeholder and leaseholders or landlords and tenants. And even flats. There'd be a company, and if necessary start a company to run this matter as it's hardly a problem to run a company. But in most cases it'd be outsourced to a private network company who'd install the leased line, wire up the properties and provide the service to all the properties.
5. The person with the leased line will be liable for any abuse which takes place from their subscribers.
If they're a CP and declared as a public ISP. But why would they need to be? Even so, this is trivial.
Openreach now add an extra charge for using their EAD links to aggregate FTTP to other properties.
If you're an altnet and a customer of Openreach's with the ability to order PIA products or Dark Fibre, yes. But you wouldn't be.