Hi, let me answer your questions afaik
1. Only needed if you plan to use your FSW in standalone mode with dynamic protocols (OSPFv2, RIPv2, VRRP, BGP, ISIS) and multicast protocol PIM-SSM. I believe it's perpetual once enabled, confirm this with your reseller please.
2. No additional licenses to work with it.
3. Using fortilink you manage your fortiswitches through Fortigate; all FSW management is done on the FGT web o CLI (or fortimanager if used). You don't need to log on FSW.
But, if access to an individual FSW CLI is required, you can connect to it over SSH or Telnet.
(SSH/Telnet access must be enabled in the MGMT interface of your FSW248s.)
4. Yes. The number of FSWs you can control with fortilink is related to Fortigate model.
If you plan to move your L3 entirely to your FortiGate and use Fortilink to manage your FSWs, probably you will not need advanced features licenses.