Short answer - You need to put SRX into transparent mode for Your requirement to work.
- pay attention to IP addressing on the diagram, please!
Long answer - the lab woes You are describing are consistent with intoducing a L3 boundary into flat L2 network without changing anything else. The issues You have are stemming from the fact that ARP resolution on the hosts cannot get thru the L3 boundary (Your SRX in L3 mode), period.
Thanks for your response!
I couldn't open the link you provided; it results in "Unexpected error regardless which browser I use. However I looked up transparent mode, and found several articles that discussed this. So THANK YOU for pointing me in the right direction!
One quick followup question (only if you know). While troubleshooting this I purchased an SRX100 online, to try to replicate at home. That device is running JUNOS 10.x. Since it's EOL, do you know if there's any possibility I can use it in transparent mode? (I think that became available in 12.x). Any way I can purchase the updated image with that device no longer being supported?
Juniper Networks Technical Assistance Centre (JTAC) could provide You with JUNOS software for EOL products provided the device is not past End of Support date.
The EOL dates for Juniper SRX products are published here https://support.juniper.net/support/eol/hardware/srx_series/
In any case, You have to have a valid support contract to avail JTAC support.
The link is: https://kb.juniper.net/InfoCenter/index?page=content&id=KB31147