First agent was incorrect. If it was a 016 ticket UA has ultimate control over the ticket. When the ticket is issued it automatically "pushes" control of the coupons to operating carriers systems to allow them to manage transactions associated with the coupon, but as long as that coupon isn't in a status that would indicate the OA is attempting to redeem that coupon against the ticket (checked in, lifted, used), UA can always pull control of the coupon back since it's on their ticket stock to void/exchange/refund it. The system does this automatically when it is reissuing a ticket, but sometimes an agent will need to manually "get control" of the coupon.

Longer explanation: the way billing is handled between airlines involves the coupons within a ticket. A ticket might be sold for multiple flights, but each flight on that ticket has an individual coupon. The ticketing carrier holds all the money for the ticket when it is sold. As flights are taken the coupons are redeemed for payment. When all the flights are on the ticketing airline, it is a moot point since your redeeming them for money you already hold. However when it involves OA flights, the operating carrier submits the coupon for their flights on the ticket to the ticketing carrier in order to receive payment. Historically, with paper tickets, this involved collecting the physical coupon from the ticket book and sending it in to the ticketing carrier (the coupon was proof of services rendered). In the e-ticket era this is all handled electronically, and "control" of the coupon means a carrier has been granted permission to modify/redeem that specific coupon by the ticketing carrier. So by the operating carrier having control, that is UA's way of telling them you can board the passenger and use the coupon within the ticket, then submit that coupon to us for payment, and we will give you the money you are entitled to based on the fare/our ticketing agreements. As the ticketing carrier (and thus holding the money for the ticket), UA has the right to do whatever they want with that coupon prior to it being redeemed, since ultimately the other carrier doesn't have any claim to UA for money until that coupon had been redeemed.

Examples: Say you have a ticket to go from ORD-NCE connecting through FRA. You will fly UA ORD-FRA and LH FRA-NCE. UA is the ticketing carrier so you paid them $1000 for this entire trip, and UA has an agreement/fare rules with LH to pay them $100 for transportation of a passenger in your booking code between FRA-NCE. UA has control of the ORD-FRA coupon and LH has control of the FRA-NCE coupon. You fly it as scheduled and LH accepts your ticket coupon from the UA ticket for transportation on the flight. After they successfully deliver you to NCE they turn the FRA-NCE coupon in to UA, and UA pays LH the $100.

Now lets throw some IRROPs in to the picture.
1) UA ORD-FRA is delayed and you will misconnect. They rebook you on LH ORD-FRA and you can still make your original LH FRA-NCE flight. The UA ORD-FRA coupon is exchanged for a new ticket for travel LH ORD-FRA and control of that is given to LH. Now after LH flys you to FRA they can submit that coupon to UA, lets say the agreed rate (since it was a last second change) is $1200, then after flying you FRA-NCE they redeem that coupon for the $100. In this case UA actually lost money on your ticket because they had to rebook you on OA last minute.
2) UA ORD-FRA is delayed and you will misconnect. Lucky for UA (maybe unlucky for you) they have an option through EWR entirely on UA. UA as the ticketing carrier can reroute you however they want, so they pull control of the LH FRA-NCE coupon and instead exchange your entire ticket to rebook you on ORD-EWR on UA and EWR-NCE on UA. There is no other airline involved, so UA keeps the $1000 you paid for your ticket, because no one else has any of the coupons to redeem.
3) LH FRA-NCE is cancelled, but LH has an option to connect you through MUC. Since LH has control over that coupon, they are allowed to modify/exchange/redeem it, so LH exchanges the UA coupon to instead put you on FRA-MUC on LH and MUC-NCE on LH. Since LH only has the coupon for FRA-NCE and provided that transport (although with a slightly different routing) they go back to UA to redeem that coupon and get the $100 they are entitled to. LH has to eat the cost for having to add an additional flight in there.
4) This is where it gets fun. LH FRA-NCE is cancelled due to a LH pilot strike (so there aren't ohter routing options on LH that day). Lucky for you, LX has an option connecting through ZRH which all the *A partners have agreed $200 is a fair price for. Two different scenarios can happen.
a. No one caught the fact your connection was cancelled and you get in to FRA needing help. LH by having control of the coupon can rebook you on someone else. They exchange the UA FRA-NCE coupon for a LH ticket with flights FRA-ZRH on LX and ZRH-NCE on LX. LX after flying that itinerary, redeems those coupons from LH for $200. LH in return redeems the exchanged coupon value from UA and gets the $100 they were entitled to for FRA-NCE from UA.
b. An attentive UA agent catches the fact your going to misconnect in FRA due to the pilot strike (or you called UA after getting into FRA) UA as the ticketing carrier has the absolute right to exchange any of the coupons on the ticket as long as the partner hasn't gone to redeem them. So UA exchanged the FRA-NCE coupon on LH for new coupons to transport you FRA-ZRH on LX and ZRH-NCE on LX. After flying you on those, LX submits coupons to UA, and receives the $200 payment they're entitled to. LH gets nothing as they didn't provide anything and no longer had any coupons to redeem.

These rules apply whether it is a revenue ticket or an award ticket. Award tickets just require a special booking code. The operating carrier would still be able to submit the coupon to the ticketing carrier for whatever their agreed upon costs are for an award fare for the city pairs.