I don't think there are any "security guarantees". What could they be?
The "endgame" as far as I understand it: The US wants access to the minerals as a compensation for the money already spent and, perhaps, to restore some of the support currently put on hold (satellite data access). Once the Ukranian resistance is broken, the US and Russia will jointly dictate a peace, gradually install a Russia-friendly regime and split the profit between them. They will happily invite the EU to finance some of the rebuilding of Ukraine that is then mainly performed by US and Russian companies. The US furthermore hopes that by spearheading the lifting of sanctions it will get priority access to some beneficial deals with and within Russia itself.