I wasn't feeling too confident with what I had heard so far. Since the gifting system has changed so recently, a lot of the information floating around is outdated, and those spreading the information don't even realize it.
There are numerous games on Steam for $0.49 that I could use to test this. Worst case I lose a few cents. So I tried it out for myself. The information and screenshots below are from the Steam client as of November 3rd, 2017.
Gifts can be rescheduled.
After purchasing a scheduled gift, you can find it listed in the Manage Gifts and Guest Passes screen. Interestingly, gifts also appear in your Steam inventory, as they did before. Even though gifts can no longer be purchased to inventory, scheduled gifts go to your inventory.
Selecting a gift does give the option to Manage gift... The person I mentioned in my question that did NOT have this button was likely experiencing a glitch.
Clicking Manage gift... not only gives you the option to reschedule the gift, but also to just deliver it now, which is the exact option I was looking for.
So what are the limitations of this new system?
- Although you can reschedule the gift, you cannot change the recipient. The only way to do this is to refund and repurchase the gift, at the current full price. This prevents you from stocking up on sale prices for friends you don't even have yet (including selling to strangers on the Internet through unofficial means). You have to know who you want to send the gift to at purchase time.
- The gift has to have a scheduled delivery date. As mentioned in the question, I intend to purchase gifts and schedule them for delivery far in the future so that I can choose the "Send immediately" option when I actually want to send the gift. There is a risk here that I will forget about the gift, and then it will be automatically sent at the arbitrary date and time I chose when I purchased it.
Unfortunately, there is no option to purchase a gift with no scheduled delivery date.
Also, the delivery date cannot be that far in the future.
Originally I selected December 31, 2018 at 11:59 PM as the delivery date, since that was the latest I could select. However, later in the process I got this cryptic error.
Changing the delivery date to 2017 caused the error to go away, so there must be some limit on the time (maybe one year). I could have tried various times until I found exactly when the error appeared, but didn't do so.
It might be possible to keep a gift indefinitely by rescheduling it farther into the future every few months, however it is also possible that you can only schedule a gift for delivery one year from when it was purchased, in which case the system is slightly more limiting. If I don't find a time to give the gift within that year, my only option would be to refund it before it's too late.