8-9 months is somewhat optimistic, it's doable but you would have to be dedicating virtually all your coupons to gaining seals, which will inevitably put you behind in power gains. I'm going to throw some reasonable numbers at you.
Wishing tree: 17 coupons. Daily wheel spin (x2 for recharging every day): 20 minimum. Monthly Card: 50 coupons. Succeeding in 1/3 plunders each day: 50 coupons. This totals up to 157 minimum could could rely on getting each day. Some days you will get 30 or 50 coupons from your wheel, or 150 coupons from plunder. Some day you may get coupons from your group wheel or other sources, but at minimum you should expect to get about 157 a day, just shy of 5000 coupons a month. Again, you will LIKELY get more, this is just a minimum expectation.
The math here shows a problem. 5k coupons over 9 months equals 45000, assuming you save each coupon. If you get every scroll you buy at a discount of 100 coupons (and you wont, often the discount is around 112 or 110, but I will assume a best case scenario here) that is 450 scrolls over the course of 9 months. You are way short.
However, there are many other ways to get those seals. Two are available from the monthly login, along with two each week in the weekly mission shop. If you are dedicated to GNW you should expect a good 20 seals each month there - or more (your mileage may vary depending on your server with that one). normally one is open each week from the chase missions, plus a handful here and there from other weekly events that pop up. There are other weekly events that can grant the seals or fragments like matsuri, but I won't include those since they really depend on your performance. With a bit of luck you could expect a good 30-40 scrolls each month with diligence in events. Over nine months of saving? That's around 270-360 scrolls, for a total of 720 - 810 scrolls through all events and coupons during the course of nine months.
If it's a current treasure (Kage) you will end up with 135 free pulls during that time as well, which should be plenty to get you over the top. Getting better discounts on the scrolls will also help increase your gains.
Overall you can expect about 80-90 scrolls monthly if you are spending around 50 dollars a month. The problem with that is that you are focusing all your rewards towards stocking up on seal scrolls. Even with that sweet super rare ninja, a player who pumped their money into power could still smack you around.