OP - You will not be connecting. You will be ending one itinerary and starting an entirely new one.
If you are late for your UA flight, there is no "flat tire" because this is intl. UA may or may not rebook you depending on its whim. If it doesn't, you are stuck. If it does, you may well wait a day or two for available seats.
Strongly agree with the above. Fly in the day before.