Worth looking at what @NickBoles says about how the Irish border backstop would be dealt with under his “Norway for now” proposal.
His analysis is here. 
The key point (or key assumption) is that - unlike the transition or zombie membership period - doesn’t need to be brought to a definite end in eg 2020/21.
That is certainly right legally: while a transition period has to be limited because the legal basis for it is Art 50 (and it is agreed that that can’t be the basis for anything other than a temporary time limited period), EEA/EFTA membership would need a definite end date.
(NB Art 127 EEA provides that any member can leave with a year’s notice. So it’s clear the U.K. could leave when it likes, subject to any backstop agreement.)
As to whether it’s right politically, the assumption is that EEA membership is at least tolerable, while transition isn’t. No CAP or CFP. No direct effect, some power to influence/adapt new EU law, EFTA Court (1/4 of whose judges would be U.K) rather than ECJ with no U.K. judge.
Those features remove the serious legal issue (which is also a political one) with the “no vote but accept EU law has direct effect” aspect of the transition agreement: that it (IMO) it probably infringes A3P1 ECHR (right to vote for your legislature).
Returning to the backstop, the argument then is that, because there is no definite end to EEA/EFTA period, the backstop is less critical. That’s right, but given that the U.K. could leave EEA at any time (with 1 year’s notice) it doesn’t remove the issue.
Nick’s idea is that the U.K. would commit not to leave the EEA while the EU was still negotiating successor agreements in good faith.
This needs a bit of thinking through. “Good faith” is an inherently grey concept: one person’s good faith issue of principle is another’s bad faith nitpicking.
So one key question is who decides whether the EU is negotiating in good faith. The answer “the U.K.” effectively makes the backstop dependent on a unilateral U.K. decision as to whether it thinks the EU is acting in good faith. Not acceptable to the EU, I think.
You could set up an impartial judicial dispute resolution mechanism. But it would have a very difficult task involving a very political judgment in a charged situation. Not ideal.
So this bit of Nick’s proposal needs a bit more work, I think. But then proponents of the current transition proposal have the same problem, aggravated by the definite end date for transition.
An important remaining point - which comes under the heading “probably soluble, but don’t ignore” - is that the EEA Agreement contains provisions about tariffs and trade remedies that won’t work for the U.K. if - as Nick proposes - it is also in a customs union with the EU.
(The CU element is of course needed now to avoid infrastructure at the Irish border as well as customs issues across the Channel in 2019 which current business supply chains and infrastructure couldn’t deal with.)
As to whether the EEA for now is politically achievable either in Parliament or with the EU27 and EEA3 I’ll leave others to comment./end
