Q1 - is this right; can this lead to a territory with mixed friendly units (e.g., US and UK) together in the same territory?
Yes, allied units can occupy the same territory.
Q2 - if this leads to a territory with mixed friendly units (e.g., US and UK) together in the same territory, what does this mean for combat?…If this territory is attacked, then who defends, all units from both the UK and US?
Yes, all units in a territory defend.
Q3 - if the UK wants to attack from this territory during its turn, can the US units assist? Or do they just lie back idle?
They lie back idle; there are not joint attacks. Only joint defense.
Q3 - Who owns this territory - UK or US? or whoever was there first?
Well, whoever originally owned it yes. If W. Canada is empty for example, moving a US unit there doesn’t change ownership. The only time you can collect on your ally’s territory is if their capital is gone AND you liberate it from the enemy. If you simply liberate it from enemy control, it goes back to the original owner. So if the US kills German units occupying Karelia, then Karelia goes back to the Russians for income purposes.