Re: "a key aim of the NIF is researching the design and maintenance of thermonuclear weapons"
>I'm not sure why the USA keeps quite so many<
Multiple reasons, I think:
(i) commitment to the trinity of delivery systems for strategic use: Air-dropped munitions, land based missiles and sub-based missiles, which in turn is partly a services rivalry thing, potentially, with both USAF and USN having weapons.
(ii) strategic vs tactical weapons (hence battlefield nukes for the US Army and smaller air dropped munitions for the USAF/USN)
(iii) redundancies to ensure a minimum number of weapon systems that actually hit the target - allowing for possible 'first strike' against the US that destroys some of the weapons; lack of reliability or lack of accuracy (yes, even Nuke's need to have a degree of accuracy) which means that some weapons fail or miss; possible counter-measures deployed by the enemy (including a what-if they have something developed in secret type concerns)
(iv) deploying them where they are 'needed' (so warheads on bases and ships worldwide)
(v) (and possibly the main reason) Because 'our nuclear stockpile is bigger than your nuclear stockpile': I think more a 1950's - 1980's mentality than now, but it leaves a legacy - we need to replace these weapons, not just scrap them, because if we used to need them, we obviously still do.