To be clear, Constitutionally the United States Senate only needs a majority to pass a law that changes the Taxes collected from corporations.

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.

Reconciliation is a process within Congress by which the Senate does not impose its not-Constitutionally-required supermajority requirement to certain budget-impacting measures.

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.