The Senate Filibuster which requires a 60-vote majority is not a Constitutional provision, and you wouldn't strictly need a Treaty for this agreement, if all the countries simply adopted the same tax provisions.
So, if the Democrats had the vote for it, they could pass this law with 0 Republican votes. In fact, since raising taxes is a budgetary measure, they could absolutely pass the corporate tax increase on reconciliation and do it with 0 Republican votes and without touching the filibuster. So, it seems quite plausible to me that this will happen.
So, no, its use doesn’t bypass Constitutional powers (not rights, which are not attributes of government bodies) of Congress, it is a means by which Congress has chosen to exercise them.
If the Senate passes a law that changes the corporate tax rate to a certain amount, that is a budgetary measure that could absolutely be passed with reconciliation.
Yes, this law wouldn't be a treaty, but it could have a similar effect.
If it was revenue-neutral, sure. That is unlikely to be the case with changes to corporate tax rates. And even then, you are going to have a hard time getting even 50 votes.
No, revenue-neutrality doesn't weigh in favor of being eligible for reconciliation; a measure must principally address either spending, revenue, or the debt limit to be eligible for that process.
One of the budget reconciliation categories is explicitly for revenue, which means being revenue-neutral would make it harder to pass under reconciliation.
Being an aspect that explicitly impacts revenue makes it much easier to pass under the revenue reconciliation process. In fact, adjusting those rates would be a pretty straight-down-the-middle use of reconciliation.
https://en.m.wikipedia.org/wiki/Reconciliation_(United_State...
Which is among the reasons this won’t technically be a treaty in US law (even if it is in international law), but a Congressional-executive agreement [0].
[0] https://legal-dictionary.thefreedictionary.com/Congressional...
https://www.brookings.edu/blog/up-front/2021/02/05/what-is-r...