From what I am reading the UK now has the freedom to either continue with alignment in the future or not, if trade with the EU continues to drop in the future and it is possibly in our interest not move the same way as the EU, we have freedom to do so. Irrespective of zero tariff/quota it’s not a guarantee that the EU will continue its market share.
That's also the reading of D. Davis, who said sovereignty lays in the fact the UK could decided to deviate, but that it likely won't do (much) anytime soon, given the importance of the Single Market to the British economy as well as the consequences of any significant deviation by the UK to the very important zero tariff/zero quota arrangement.
Which leads to the question you've also touched on briefly: as long as the EU's importance to the UK stays what it is, it makes little sense for the UK to deviate much from the EU rules, so the right that seems so precious to Brexiteers is not going to be used significantly anytime soon and is thus largely theoretical and symbolic only. Yet in return for an illusive right, the UK looses the practical right to hold the pen for any future EU legislation which the UK will mirror for the forseable future anyway, a degradation to vassalage status which is very real and almost immediate!
You can rest assured that it won't take very long before the first major issue arrises with the UK having to choose whether it will follow a new EU rule which it doesn't like at all but can no longer block at the EU level any longer. So it will soon and often have to make the binary choice to just swallow the EU rule anyway by rolling it over in a British copy-paste law or (risk) ending this whole agreement with the EU for just 12 months later! That's exactly the situation Switzerland is in, where MPs in Bern can discuss for hours all the merrits and risks with each EU directive send over to them, but ultimately are always finding that they push the green button because not doing so triggers the guillotine clause in their bilateral and kills the Swiss economy on the spot.
Knowing that if
the importance of the EU to the UK would somehow dwindle to the point that it is dwarfed by that of others (combined), the UK could always have left it undisputed, unhindered and totally free of any further commitments in just 2 years time under article 50 of the TEU, so if that is they whole logic underpinning Brexit, then it comes some 50 years too soon, going by JRM's comment in the House of Commons saying the benefits of Brexit would only be seen in 50 years or so...... better have waited a bit with Brexit then and continued to take full benefit of the UK's truly unique and bespoken EU membership for the time being and if
the economic conditions would have been right to do so, left the EU only then.
I only sell my car once the new one has arrived, yet Brexiteers have sold theirs while they haven't even signed the order form for their new one as it still needs to be designed!
And so they will now have to go on foot for the forseable future, making things up about how much healthier this is and how much more time they'll have to enjoy the scenic landschape along their way to work... not mentioning they'll show up late everywhere, often soaked wet. That's Brexit for you in a nutshell for the next 50 years at least.