If two parties have agreed upon an arrangement
before the treaty came into force, that arrangement stands unless
otherwise stated in the treaty.
You should know that as China is supplying Nuclear reactors to
Pakistan in theoretical violation of NPT under the grandfathering
clause of Chashma reactor.Since the Trident deal was agreed upon
before MTCR came into effect, it does not place any restriction on US
to transfer Trident II.
And to add to this, MTCR does not have legal penalties. It is bound
together by the understanding that if one violates MTCR, others would
do so jeopardizing the interest of that country.
Condor II was not scrapped due to MTCR but due to US pressure as it
had involvement of Iraq (Gulf War).
UK's nuclear weapon status was mentioned to counter the assertion of
it using US warheads on Trident II.
Since India was a victim of several sanction before, I don't have to
remind you that how many those so call agreement that was signed
before sanctions was not honored after the sanction. As you pointed
out yourself since MTCR has no legal penalty, for the trident deal, no
one is putting pressure on US or UK to terminate that deal where as
othe nations such as Argentine, Brazil, South Africa and Taiwan were
force by others to eliminate their projects.
UK's nuclear weapon status has nothing to do with its ability to
design or manufacture MIRV warheads because miniaturization of
themonuclear warhead is always the hardest part. So one can safely
say that without the assistance from US for the warhead UK's Trident
D5 missile might end up equiped with only a single warhead design
rather than MIRV as it carries now. Talking about proliforation of
weapon of mass dstruction. That is why UK government has been denying
any accusation of this warhead had anything to do with US until
someone leaked it to the public.