I could say the same about your repeated invocation of a Latin phrase that you seem to be throwing around as if it is a magical incantation rather than a legal principle that has specific limitations. On Article 50, just read it - it is explicit and quite clear. Paragraphs 1 and 3.
1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements. .....
3. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
This is clear and unambiguous.
The article nullifies whatever voting rights the UK possessed within the EU from the time it is invoked, and it gives Brussels all the leverage in negotiations and the UK none.
No, it doesn't - at least not the underlined bit. I believe you're misinterpreting paragraph 4. The UK does not lose its voting rights in the EU from the time Article is invoked. It only loses its rights concerning the specific issues raised in Paragraphs 2 and 3 - and it says explicitly that it only applies in those issues (For the purposes of paragraphs 2 and 3). This is because if it didn't lose those rights, it would in practice be negotiating on both sides. All Paragraph 4 does is say that it can't affect the negotiation position of The European Council in those negotiations. It can still represent itself and it remains able to participate in the deliberations of the EU on all other issues not connected to its exit.
Does Article 50 give Britain any special advantage - only that it means that it's out after two years and a plain text reading leaves very little room for a legal case to stop it. It's explicit. Once invoked, Britain is out in two years at most unless it choose to have the time extended (and the Council agrees). That is a huge advantage when any other approach could prevent the UK leaving as far as the rest of the EU is concerned making it virtually impossible for it to negotiate any advantageous deal with any EU country, and extremely difficult with any other country that doesn't want to be forced to choose between the UK or the EU.
Rebus sic stantibus is always valid
No, it isn't. It's actually quite limited under international law - if it wasn't no treaty could ever be binding. It's limitations are described in Article 45 and Article 62 of the Vienna Convention on the Law of Treaties. (Article 62 outlines when it can be invoked, Article 45 outlines the exceptions to Article 62.)
And this may be even more limited in the case of the EU Treaties.
I've already given you a link previously to a document that outlines legal position, in my view, fairly clearly. I'd also refer you to Hofmeister, Hannes. "‘Should I Stay or Should I Go?’—A Critical Analysis of the Right to Withdraw from the EU." European Law Journal 16.5 (2010): 589-603.
To be fair, Hofmeister believes that in a strict legal sense, withdrawal from the EU under general provisions of international law might be theoretically possible but: Subject to the conditions laid down in Articles 56, 60 and 62 of the VCLT, a state may thus withdraw from the Union. These Articles, however, allow for withdrawal only in very exceptional circumstances, thus making withdrawal a theoretical rather than a real option.
And who would decide whether that was allowed? The JCEU - a body with quite explicit motivations to decide that the Treaties of the European Union supersede the Vienna Conventions in matters concerning Europe. If organs of the European Union didn't routinely display that type of arrogance, there would be far less reason to leave it.
You have way too much faith in the Treaty of Lisbon and in the EU as a good-faith negotiator, which they have proven to not be over and over. And you have no faith in a sovereign nation to stand on its own two feet.
The EU regards the Treaty of Lisbon as its constitution, even though it flouted same with respect to all the “bailouts” from 2008 and onwards, and also in several other areas. For the UK to invoke Article 50 of that entity’s constitution means that the UK is not reasserting its sovereignty, and to ignore rebus sic stantibus is to gloss over the EU’s violations of its own constitution.