Clarification on EVPN-VXLAN L3 Gateway for vlans/vni's....Juniper training

Thanks to the users who answered some of my noob questions earlier this week.

My company bought an account with all access pass from Juniper and two of us are doing the DC course. Here is an imgur link to a screenshot from a slide where they configure the gateway on the spines for the vlans ar IRB sub-interfaces.

1. What's the difference in design/functionality when the gateways are on the leafs vs the spines? (Nothing plugged into the spines except leafs)
   
2. If you have 300+ vlans in a data center and 30+ leafs, won't that be a heck of a job to either put all L3 gateways on all leafs or keep track of which leafs need which gateways since not all servers are on all vlans on a leaf pair?
   
3. If you had to migrate from an legacy 3-tier Juniper/Arista network to a new Juniper fabric, how would ytou transfer the L3 gateways on the previous legacy distribution layer to the spines in VXLAN?
   

Thanks for the help mates.