You've slipped in a bit at the end of this post, so I'll answer it. Legal obligation is highly relevant when discussing the divorce bill. As guardians of the nation's purse strings, the government has a duty not to give away monies that it is not legally obliged to. However, in discussions over a future trading relationship with the EU, goodwill payments can properly be considered because the UK will gain a benefit (as will the EU) from smooth transition and friction free trading going forward.
That's why it's so ridiculous of the EU to compartmentalise divorce and future trading. That is what's causing the problem. And because the EU has given itself the unilateral power to decide if and when a future relationship will be discussed, the ball is firmly in their court to get things moving, if businesses all over Europe are not to suffer badly.
How much we pay is a political decision. The politicians may choose to hide behind some legalistic smokescreen if it suits them, or choose to ignore it if it suits them. Besides, the ECJ trumps some House of Lords Committee as long as we are still in the EU, if you want to go down a purely legalistic route.
It seems we are involved in a game of international chicken. May started this with her highly adversarial kick off for the British position, when she thought she had authority. As I have said before, in purely economic terms the EU has already resigned itself to being worse off as a result of Brexit, but they know the UK will be far more worse off, especially if there is no deal. They probably reckon our government will blink first. I think they are wrong, for several reasons. We don't actually have a properly functioning government, just a zombie PM and a highly fragmented cabinet with members who contradict each other on a regular basis, all resting on a wafer thin cobbled together parliamentary majority. They are not operating on any collective, rational basis, just on individual self interest and an obsession with what things look like to a certain, shrinking, segment of British voters. We will stagger on reaching no decisions, not because we have a clever strategy, but because we aren't capable of reaching collective decisions. They way things look at the moment I think there is a strong chance that everything will break down and we will have a chaotic cliff edge Brexit. Just because common sense and rationality argue that both sides will see sense to avoid this is no guarantee that it will happen.
And then we will say it's all the EUs fault, and there are plenty of people here who will believe that and that a catastrophic political failure shows that are in fact strong and resolute, **** the job losses, **** the inflation, **** other people's suffering, we put the Great back in Britain (ignorant of course that Great in this context is a descriptive geographical term).
Of course you will disagree with all of this, as is your right. What mystifies me is how Brexit supporters, of all their various positions, seem to be giving the government a free pass on all this. They are serving everyone, Remainers and leavers alike, appallingly.
Addendum: a couple of weeks ago the Government published a 14 page paper on future customs arrangements with the EU. Pride of place was given to a proposal to electronically track goods which would mean no long waits at borders. Businesses on both sides of the channels were both baffled and sceptical about this 'innovative' proposal.
Yesterday, in the USA, David Davies told a bunch of American businessmen, that actually, there would have to be a hard border with goods declared and a good proportion of them physically checked, in both directions, because the 'innovative' proposal won't work. Apparently everyone is now working hard on minimising the anticipated delays (the obvious solution, stay in the Customs Union and Single Market, may occur to him at some stage).
And we call the EU vindictive because they get pissed off with our government presenting them with patent hollocks, then changing its mind. All of those position papers were full of the same flimsy, ill thought out bullshit. Amateur hour. Either the civil servants who are supposed to generate these ideas are as thick as their political bosses or they are being ignored.
While I am here, let's get the no deal consequences clear. We are an import dependent economy, as a glance at the balance of trade figures makes very clear. The £ will fall even further in the event of no deal, making our exports even cheaper (including tariffs). Meanwhile the imports, which we still need, will be more expensive because of the currency position and tariffs. This includes materials for the exporting businesses. So the net result is inflation, as always hitting the poorest, and lower margins for exporters. Oh yes, it also means losing the trade deals with 50 countries that we already have through being a member of the EU.