I dint want to seem like I'm constantly contradicting you, but on this last point. The financial settlement formed part of Phase 1 of the negotiations along with the Irish Border and EU nationals rights.
Joint report from the negotiators of the European Union and the United Kingdom Government on progress during phase 1 of negotiations
ec.europa.eu
Also the UK signed up to the Multi Annual Framework in 2014 and committed itself to that until 2020.
Find an overview of the current and past EU budgetary system, plus funding opportunities, latest news, results, and figures from the budget department.
ec.europa.eu
So the UK is both legally and morally (good faith principles) obliged to pay that money. Also Section 3 of the European Communities Act gives the ECJ precedence in the UK, which it won't continue to have in the event of 'no deal' but then another arbiter likely International Court of Justice or the Permanent Court of Arbitration, has to make a decision on what ruling it will give on any money owed, which could amount to much more owed by the UK than any settled agreement.
Now the argument has been by Rees-Mogg and others that the Vienna Convention on Article of Treaties releases you from any obligation of ongoing performance of a treaty once that agreement is cancelled, however the case law is largely against that as it requires 'unforeseen' circumstances.
Also the EU would rely on obligations made by the UK under the EU Aquis Communautaire which are pretty clear and there is a prosess for withdrawal mapped out under Treaty on European Union.
In short, if you commit to a joint multi year finacial and political agreement and you decide you no longer want to operate with them, there has to be a division of assests or financial settlement to that and unless you agree this first, then you cannot enter into discussion about what sort of relationship you want to continue to have in the future.