I think you are having slapbacks due to not fully aborting the attack.
I had the same experience when I started using oxygen. I would get hit at night, 10 to 15 min on O2, and back to bed. Repeat every 90 minutes all night. This is not my usual pattern, I get hit at night, but usually only once or twice a night. This went on for a few weeks, and I thought maybe O2 didn't work for me. I was even starting to think O2 was making things worse.
I was experiencing slapbacks because I hadn't fully aborted the hit, even though I was pain free after a few minutes on O2. I bought the special Cluster mask, which was well worth the $25. I increased my flow rate from 15 to 25 LPM and increased the time to 15-20 minutes and then the 5-10 minute cool down (10-15lpm) at the end.
I am not an expert by any means, but I would recommend increasing the flow rate. Sometimes I get worried about my oxygen supply and cut back the flow rate to conserve, and I regret it every time. I can abort with 15lpm, but I sometimes end up back on the O2. I have a gigantic H tank at home (the O2 delivery guy hates me) and an E tank for the car. I never play with the low flow rates anymore (always 25LPM) on the E tank because I am at work and have a fear of running out of O2. I might use more O2 the first time, but I only need one trip to the parking lot.
I understand your desire to conserve oxygen. My insurance won't cover it, and it can get expensive. You might want to look into welding oxygen. It is the same quality as medical oxygen, but comes in a different tank and is usually cheaper. I have used welding oxygen in the past, and would not hesitate to use it again. Also, that big H tank of medical oxygen actually comes in a welding tank that is painted green.
Another idea I could suggest would be supplementing the O2 with an energy drink containing caffeine and taurine. This has helped me abort daytime attacks. However, I haven't tried it at night. I have read from others on this board that they can drink an energy drink during a night hit and still go back to sleep.
Above all, I would recommend not giving up on oxygen. Increase the flow rate and try to find an alternate (cheaper) supply. You have proven you can get pain relief from oxygen, now you just have to dial it in to prevent the slapbacks. You should get bigger tanks, too. Those E tanks are awesome for the car or travel, but the big boys are best for the house. My oxygen delivery company actually charges me the same price for the H tank as the E tank (which makes no sense to me, but works out in my favor).
Congratulations on kicking out the triptans. I did that a few months ago and have felt much better. I can't prove it yet, but I think it has reduced the number of attacks I get.
Also, congrats on kicking the topamax, the worst drug I have ever taken. I'm glad it didn't help me, because the side effects were insane. It would have been a hard choice if it actually helped.