1) The only reason why this time fukurukumaru deal had no ninja is because was together with the introduction of edo hiruzen. Do not worry, next time you'll have back ninja fragments.

2) recruitment feedback was depowered a ton. Used to give 54 7days scrolls for 300 used, now grants 44 for 300 used. If you want to know if is worth to go for the 500 option that grants 2 edo hashi fragments you have to make a comparison with what the other rebate grants you with 500 seals. that one grants you 66+36=102 seals and since in average you get 1 fragment of hashi every 40 seals used you have your reply. The 500 option is worth it only if you desperately want edo hashi and you not care about anything else, but the advantage is limited to the 22 more scrolls (102 vs 124, where i consider the 2 hashi frags worth 40 scrolls each one) that need to be necessarily used within that week so you cannot use them for the eventual following rebate.

3) until you are able to keep 200 3-days activity rate stay in your current group and farm easy gnw packs. If you go in another group the packs do not multiply.

Saving more than 35k coupons make no sense, there is not and there will never be an event where would be worth to spend the coupons in excess after the 35001st.