To be quite honest, I'm not sure if the inline-jflow supports version 5 flow records, but I tend to agree with you. If someone could 100% confirm, that would be great.
As for your question about how to enable the RE-based sampling, it looks to me like you have the minimum config in there, as this is what's in some of our MXes:
set forwarding-options sampling sample-once
set forwarding-options sampling input rate 1
set forwarding-options sampling input max-packets-per-second 7000
set forwarding-options sampling family inet output flow-inactive-timeout 60
set forwarding-options sampling family inet output flow-active-timeout 60
set forwarding-options sampling family inet output flow-server x.x.x.x port 2055
set forwarding-options sampling family inet output flow-server x.x.x.x source-address y.y.y.y
set forwarding-options sampling family inet output flow-server x.x.x.x version 5
And we're doing firewall filter sampling on the devices with this configuration, as opposed to interface-based sampling. Unfortunately, I don't have access to an MX104 to test out your config.