If all I need is L2 bridge would there be any reason to keep arp enabled on bridged interfaces?
Similarly, why proxy-arp is available for interface that is a part of a bridge - isn't it redundant
As soon as you add interface to the bridge forget about any options of those interfaces - only bridge options matters. If there is no IP on the bridge itself and there is no need to communicate from the bridge to any other device - you can disable arp
yep, all those settings are necessary only if you need routing between that bridge and some other interfaces. for true bridging, they don't play any role
basically, bridge simply sends packets from one bridge port to another ports of the same bridge. as soon as bridge receives packet with its MAC address as destination, packet either goes to the router itself (if dst-IP is router's IP), or is being routed to another interface (ethernet, vlan, another bridge, tunnel, etc.)