Hi,
I think the best is the solution A (buy it under the client name).
But you can also use the solution B and if your client need his independence for the updates etc ; we still have the possibility to migrate the license to another account (with some proof for sure).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.