CG, sorry, but you took tried to solve the problem the wrong way.
Admitting the goal is to half the time to promote an old character to 7*, you decided to double the shard won...
Ok, but then you saw, we must adjust SSC, and then old and new characters don't have the same conversion value, and it become more complex than it should be.
My proposition, instead of all this, half the needed shard for old characters, no other changes needed for shard shop or anything else.
So, instead of needing
10 + 15 + 25 + 30 + 65 + 85 + 100 = 330 shards to reach 7*
you only need for old characters
5 + 8 + 12 + 15 + 33 + 42 + 50 = 165 shards to reach 7*
I've only small hope this proposition is not lost in the rest of this thread, but I had to do it, because I think it would have been the best way to achieve the goal.
Edit : corrected a number