My responses are below in BOLD.
1. What benefit does providing a list of arbitrary units at a base address provide to the user who is typing in the address? It would seem to me that if the user didn't know the unit that they would like to find, providing a list of units at the base address provides little benefit to the user but maybe some clarity on how this is beneficial to the user would help with this.
To @MattFancher1s point, alerting a user that subaddresses exist at this location is they key thing we'd be looking for. Subaddress information is not always that widely known or easy to find outside of people very familiar to a property. There are business workflows out there where just the mere suggestion that subaddresses exist could change a users approach.
In a public safety sense, lets say a dispatcher is on the phone with someone and they say they are at 123 Main St, type it in, and now the dispatcher sees subaddress unit suggestions showing up; it could then prompt the dispatcher to ask "what unit are you in" and potentially narrow down to a more accurate location for the caller. Customer service may send a bill to the base address 123 Main St, and be completely unaware there are subaddresses that exist and now they sent that bill to the wrong person.
2. What order should the results be in if we were to support this workflow? Should they just be the first 5, 10, 15 units at the location (depending on the max suggestions setting)? If so, it would seem that the user would only start getting suggestions they cared about after entering in some information to whittle down the suggestions to something more applicable.
Just the mere presence of subaddress suggestions is probably enough and the order doesn't probably matter that much, it would likely be more specific workflow dependent at that point that I think would be difficult for you to account for. With stacked Address Points that have subaddres information, I do think Reverse Geocododing will actually spin out a response of 123 Main St, Unit 1 - Unit 300, or something like that, so even that could be an alternative to listing everything??
3. If the user was to set the max suggestion setting to something quite large, I would argue that they may spend more time scrolling through the suggestions to find the unit that they are looking for instead of just entering a character or two to get suggestions that were more applicable.
Again, it's largely just the mere suggestion that subaddresses exist at this base address is what a lot of us are asking for. Now that a user knows subaddresses exist, they can start typing 1 or 2 or 5 or A or B to narrow down the suggestions at that point.