As u mentioned by yourself u can't use FTD with FDM on Site B because it doesnt support PPPoE. So install an FMC in Site B.
The real problem is, that u have to configure a Site To Site VPN under Devices, where u can add vpn (firepower threat defense device) and choose point to point and under node B Extranet where u have to add an ip address. See the Problem? u cant use an dns and the endpoint on the other site gets an ip via pppoe, so it can change on every reconnect via isp.
In other words your config wont work if site A and site B dont have static ip addresses.