Memorandum is non-binding.
The Budapest Memorandum was negotiated at political level, but it is not entirely clear whether the instrument is devoid entirely of legal provisions. It refers to assurances, but unlike guarantees, it does not impose a legal obligation of military assistance on its parties.
[1][46] According to Stephen MacFarlane, a professor of international relations, "It gives signatories justification if they take action, but it does not force anyone to act in Ukraine."
As far as NATO membership, there is a requirement for new members to have a settled territorial disputes. Which means admission can't happen untill Ukraine either outright gains control of DNR/LNR/Crimea or they conceed some of that land to Russia.