If you have 1 Site(A) to site(B) with SRX on each side and wish to add another link from one(SITE-A) SRX to a new device but to the same site(SITE-B).
Yes, that should work.
Only point is, in phase2 use proxy-id as compared to Traffic Selector as the latter would add an automatic route for the remote subnet which will be same in this case.
For the failback, once correct tunnel st interface comes UP OSPF route will switch the traffic back.
Yes, using OSPF and link cost on dual vpn for failover is a good choice.
You are correct that the IPS cannot help with encrypted traffic for inspection.
IPS is most valuable for sites that allow direct access to the internet from clients and inspect this traffic. Or have resources published to the internet at the site that can be inspected.
In order to create two tunnels at least one site has to have two different ip addresses to create the vpn.
If there is only one ip address on both sides there is no way to create two phase 1 tunnels.
You can have both tunnels on the same SRX and if you want hardware redundancy the general way to do that is to create a cluster with two SRX that then statefully failover in the event of any hardware based issue. This will be less disruptive of active sessions than two independent SRX.