1) In the network diagram, a couple of the links are dashed. Because of the way these will be setup, are the dashed links necessary?
If you are going to be doing a HA cluster then these would be clustered links, if you are going to make the intermediate line between SRX's an L3 link is also ok, but a L2 link would steer away from it.
2) We will be hosting certain services on servers behind the firewalls. To avoid policy/session mismatching and asymmetric routing, is going to an active/passive situation better? (we might even have to go A/P, anyways) Is VRRP or default bgp route more preferrable?
if you are going to be doing a very basic active/passive setup with static default routing then the failover is very fast.
if you stick L3 on top, like routing protocols, then failover takes substantially longer as the protocols essentially start from scratch and have to peer up and converge
3) I was just planning on keeping both firewalls separate, each with their own [relatively identical] config. I've played around with HA on J-series routers, and it seemed kind of clunky. Would doing high-availability really prove to be that beneficial?
you end up with one config that is mirrored onto the passive node, as long as your performance and port requirements are met is a good backup solution.
4) Any other suggestions, warnings, or red-flags?
be careful with the srx software version as it does tend to be flaky, go with recommened version unless you have a specific requirement.