1. Do I need to define IRB L3-interface for each VLAN on every switch or just the core which will be doing the inter-vlan routing?
No, if you extend the layer 2 vlan to all the switches only one layer 3 interface in the entire layer 2 domain is needed to activate inter vlan routing.
2. Do I at least configure RB L3-interface for Management VLAN on every switch? This is how I intend to manage the switches.
Yes, if you want to have irb as the mgmt interface than each switch will have a unique ip address assigned for mgmt typically in the same vlan.
3. If I want to restrict internet access (default gateway) on one of the VLANs, how do I do that?
You could block the vlan subnet on your internet firewall via a policy that denies internet access to devices in the desired subnet.
4. Can I setup DHCP server on the core switch for each VLANS?
Yes the ex can be a dhcp server
https://www.juniper.net/documentation/en_US/junos/topics/topic-map/dhcp-for-switching-devices.html#id-configuring-a-dhcp-server-on-switches-cli-procedure
5. When I try to delete the irb interface for VLANs that I defined in access switch, I get following error:
'l3-interface irb.48'
Interface must already be defined under [edit interfaces]
error: commit failed: (statements constraint check failed)
To remove the interface you also need to delete the reference in vlans
delete vlans support l3-interface irb.48
delete interfaces irb unit 48
6. How do I delete or undo configuration changes made by set command, e.g.
user@switch# set vlans support vlan-id 111
user@switch# set interfaces irb unit 111 family inet address 10.0.0.X/8
user@switch# set vlans support l3-interface irb.111
How do I untie vlan 111 from irb.111 and delete it?
delete vlans support l3-interface irb.111
delete interfaces irb unit 111