You're over thinking this, IMO.
> If GV needs to call to verify the number, then I cannot use my phone with my original SIM card until after I get the verification call correct?
No. You do not sit around waiting for the verification call. The verification call happens when you go through the GV porting screens, and you control when it happens.
The GV porting signup consists of several 'enter your name etc.' and agreement screens. After some 'check your number for eligibility screens', you are asked to agree to several things in a list, each with a checkbox, then you get to 'Phone Verification' screen, where, since you've already passed an eligibility check, GV calls your number, you answer, and you must enter a code shown on the screen. This is the verification call, others on here also refer to it as the one minute you need on a T-Mobile sim card, (and any remaining minutes on that sim card are lost/forfeited when the T-Mobile account is closed in 24 +/- hours due to the port).
Put your good sim card back in the phone after you do the verification call, you don't need the pre-paid sim in the phone anymore. In 24 hours you'll get an email message from GV, and in what appears to be standard (based on ongoing reports on here) when coming from T-Mobile
GV will say your request to port has encountered an error, password or prepaid pin required. Which is confusing for first timers because GV never asked you for a pre-paid pin during the GV porting/signup. You enter the required info, and in anywhere from a few minutes to 24 more hours you get a message from GV saying porting successful.
Report back here on how it works for you,
in the number porting to GV, my experience thread.
Edited to clarify a few points.