Hi:
This has been covered thoroughly, both here on the forum, and on the Google Voice forum, for years.
Google continues to actively support Google Voice and is making continued enhancements to the service. The infrastructure that runs Google Voice has been substantially upgraded in the past few years, and it is also used for Hangouts and for Google's other telephony services, Project Fi and Google Fiber Phone. It's not going away.
Regarding porting, you can first enter your land line number into Google's porting dashboard, to see if Google's carrier can accept the number for porting. You will get an error message that Google doesn't support your carrier. That's to be expected. But, if you also get an error message that porting isn't supported in your area, then the number isn't portable, regardless of it being a land line or mobile number. Here is the link:
https://www.google.com/voice/portingThe number one and number two reasons that ports fail are because people input incorrect account information into Google's porting form, or don't wait long enough after the land line --> mobile port before attempting to port into GV. Be patient, and give the first port a few days to become fully operational and for the change to be broadcast out to the telephone carriers. I suggest using either T-Mobile prepaid or AT&T GoPhone as the intermediate carrier. Do NOT use one of the cheapo MVNOs like Lycamobile or H20. They are incompetent at porting and will likely screw up the port. You will not lose your number.
If you have questions or issues about your port, please post over on the Google Voice Help Forum, where we'll be happy to assist you.