Last question first - Sure, you can ride more miles than the minimum for a cert. Generally speaking, if the mileage qualifies for a different cert, you'd submit for the more challenging one. You can do in-state rides with any ride. I.E. An In-State BBG or BB, they don't have to be all SS1K rides.
Spotawalla is not accepted as 'proof', it helps document the ride, but your receipts are your proof. If you had an irregularity during the ride, missed a receipt, (except for a start or finish), then Spotwalla would help show you did what you say you did. I have no idea what the IBA thinks of Rever or other tracking set ups.
You don't have to do loops or out and back rides for certificate rides. But, YOU said a NV1000, so you're limited to staying within the confines of the state for any In-State ride. In most cases that's going to be a loop or some out and back, etc to get the mileage.
Wandering all over on small scenic roads is fun riding. But doing that and making 1000+ miles in 24 hours means you don't get to stop and explore anything, and every state looks the same in the dark. Never mind forest rats at night in NV, black cattle on the road, etc.
The main thing about cert rides is you need to document your ride. That means a valid DBR at the start, corners and turn around points so that the verification teams can follow your path and see that you didn't take a short cut. The route is completely up to you, but going out and back to Owyhee is a bit more challenging to find a DBR, especially after 7 pm when the store closes. And getting a receipt in Austin is easy, but there isn't a damn thing on 722 and Middlegate sure isn't 24 Hr and I have no idea if they give a good DBR. Great burgers though. I'll be on that road 18 days from now and have to use a picture to document I was there, but it's not for an IBA ride. What hurts your route is you don't follow through. You're riding out a road, then in the middle of no where, doing a u-turn and riding back the same way instead of riding the road to a destination where you could get a receipt.
Bottom line, if you want the certificate you have to be able to document that you did the ride in the manner in which the IBA lays out. Hoping to fill in the blanks with a Spotwalla track may help, but on a route as convoluted as the one you originally showed, (Google Maps links work better for showing your route than a screen shot, btw), you would be wise to submit it to the IBA for approval first.
Check out
RideMaster for events like the Alamo Express or Lobo Loco rallies. The Alamo Express is a BBG event and has suggested routes that are proven and have known documentation points.