So my takeaway from all this is, if I have 2 passengers, one with two future travel credits, the other with one, and a farelock, I shouldn't even bother trying to pay for it on-line when it comes due, but rather call on the phone?

I'd say Yes. On the Farelock purchase screen you're presented with two button options (not checkboxes, buttons, so they are mutually exclusive). One for credit card/payment types. The other for Electronic Certificate. (Note the word Certificate vs. Credit). If you choose the Certificate option, you are then prompted for the 10-digit certificate # and a PIN. The FTC has neither. So you'll have to call in.

Note too, that there's no guarantee that the agent can/will convert your FTC to an ETC to then use it. Worst case, they say "no" and you HUCA.

Is it possible to get the future flight credits converted ahead of time? Is there a downside to doing so?

It is against policy, occansionally will do for an immediate booking like the prior poster noted

If the fare I Farelock is miles + money to pay for missing miles, and I subsequently earn/transfer in the required missing miles, will the Farelock allow me to pay in-full for miles rather than the money it shows for buying the miles?

Any known issue with purchasing Farelock right now? Been trying to lock a mileage fare for 4 people for the last 24 hours and I keep getting "An error occured please try again later". It's not a CC issue as I've tried a few.

Have you tried a different browser?

Yeah tried both browser on my computer and app on my phone.

I have the same question...did you figure this out? I am waiting for Chase transfer (one of the lucky non-instant transfers). I'm able to do FareLock but the screen concerns me as it seems to be locking me into buying the miles I still need, whereas I am just waiting for sufficient miles in my account.

I've been fascinated by the website glitches re: FareLock mentioned in this thread. It has helped to know a lot ahead of when I plan to purchase the newly FareLocked trip for one way, SFO to MXP connecting in ORD, and the majority is in PP class. I'm booking about 200-225 days out and can't believe I can go on a Saturday, lower if it was mid-week, for only 90,700 miles. My CC statement closes on Monday so I'll see more miles applied plus I'm purchasing sale miles+bonus. I've been cautious about miles speculation with choosing a 14-day FareLock. The only way that I could purchase the FareLock was with a call to customer service. I connected to Shirlynn, or at least that's what her name sounded like. My theory is you're connected faster to a CSR if you opt-in on the survey, which I did. She helped me lock it for 90,700 miles. She said that she added my FF number. I logged in using the chrome web browser and noticed that the trip didn't have my FF number. I added it and I selected seats at the same time because the UA phone app wouldn't let me select seats without a response of "Try again later". It shouldn't be this much work.

In the end, I gave the CSR a "5" rating for all of the questions because in the middle of my call, my home experienced a power outage and she was so understanding. I panicked because I couldn't hear her on the phone but she was still there so it was just a coincidence that she went silent at the same time as the lights went out... It happened at 7:30 AM without having had coffee.

In summary, I haven't seen any carrier's website function without inconsistencies or errors so badly occurring so much as it has become so apparent lately. I'll spare you all the differences I see on other sites for other carriers. IMHO, the development of the carrier's phone apps complicates the functionality of the web browser version, and vice versa.

If I purchase a ticket that is being held on FareLock, will I still have a 24 hour cancelation window that starts from the moment that I complete the transaction?

Yes, the 24 clock does not start until you purchase the flight(s). Note the farelock fee is non-refundable.