How do I phrase usability questions to get useful data

Exec. Summary (or tldr): How do I phrase my questions to get the best data?

(This is a follow up to this question.)

I have a very long, paged/saveable form, used by banking agents to onboard businesses. The agents are proficient in a 90s version of this form already - we are upgrading the application to the 21st century.

The form has phone number fields as well as date fields. Currently, the fields are componentized: [MM] [DD] [YYYY] or [XXX] [XXX] [XXXX] Ext[XXXX]. But I'm not sure that's very efficient. It may reduce error but I suspect that it does so at great cost to speed. Remember, these are trained users. For example, it might be better to have the option to copy/paste whole strings of phone numbers, say, from a document they already have.

I think a full-blown usability study might be less useful than simply asking the users. So I'm writing up some questions in anticipation of asking them.

My question to y'all is this:

How do I phrase the question(s) to get the most informative responses?

Should I lead them toward opportunities they might not know are available, or should I not bias them, to get answers that aren't just re-enforcing my own expectations? Both have problems. Examples:

  • "Considering both speed and accuracy, what way(s) for entering dates/phone numbers are/would be most efficient for you?" (Open-ended and unbiased but also not helping them explore possibilities)

  • "Would the ability to copy/paste whole number strings at a time into a single field be useful?" (Helpful but leading them down a path)

  • "Do separated fields provide the accuracy you need without sacrificing speed?" (neutral)

  • "How do you enter these fields now?" or "How do you expect to be able to enter these fields?" (Not useful, since they don't have options or expectations in the current app.)

Of course I also don't want to inundate them with questions, so I have to be judicious with them.

How would you handle this?