Basic ?s first:
1-You've looked at this, yes?
http://www.obihai.com/porttutorial.html2-Are you certain, or are you in the leap of faith category, that GV currently supports ports from the NXX exchange where your grandmother's landline is?
3-Three days total is about right for a landline > mobile > GV port. The few I've done were all using T-Mobile, so ymmv.
> worried about... not having her land-line phone for too long.
During any of the ports, landline > mobile or mobile > GV, you're gonna have at least one, and for a short period of time two, phones always working. The trick is when the mobile > GV port happens is to get the obi up and running and plug in the house's phone cords asap.
I ported a landline for some elderly relatives, and one thing I took away was give them some benefit of the doubt to understand what's going on, I 'dumbed it down' too much at first. Different circumstances might warrant a different approach, alzheimer's and related conditions, etc.
One more suggestion, since you're using an Obi110. Do a several month trial period where you keep your grandma's landline and configure the obi110 with a new, free GV line to handle all outgoing calls that are not local to the exchange, and all local calls go out the landline. The landline incurs absolute minimum charges, no regional toll or long distance. If after three months all is well, port the landline and have all calls go through it. In hindsight this is what I'd do, to test if the internet connection is solid, call quality is not choppy, etc.