If an SSE owner could be prosecuted as you say:
if there is ever an inquiry into the firearms history or origination, the previous owner, and both FFL's that had part in the transaction can and will be prosecuted. So if ever the said firearm is stolen, used in a self defense situation, crime, or simply checked by a law enforcement officer and it isn't still in the SSE formation everyone involved is screwed.
then I would currently be in the middle of legal proceedings... because I have had a non-roster / SSE handgun checked by a Sheriff-- all that came up on his terminal is that it was registered to me. After everything checked out we chatted for a bit, he said "have a nice day" and I went back to shooting.
Once a handgun is DROS'd the owner can do whatever work or modifications they see fit to perform so long as the final product remains in a legal configuration for CA. If that were not the case, then why couldn't they prosecute for *any* custom work done to a handgun after the DROS that alters its configuration or parts that weren't present at the time of DROS?
The SSE is only a "workaround" for DROSing new handguns that are not on the safety roster if the purchaser does not have a valid LE exemption-- a technicality that some legislation is trying to remove this session thanks in part to some individuals on the Sacramento & Roseville PD who were using their LE exemption to buy and sell a large number of non-roster handguns at a profit. They were OK for a while until they DROS'd some stuff out of order and the CA DOJ noticed and took a closer look. Thanks in part to their shenanigans there is an assembly bill AB-169 on the table this session that would affect all non-roster handgun transfers (even used PPT sales), SSE exemptions, and LE exemptions.
There have been some concerns voiced on Calguns about non-07 FFL's doing these SSE conversions; some contacts say the FFL needs an 07 to perform this work while a new FFL in Sacramento was advised by the ATF they didn't need an 07 to do SSE conversions because they were all "drop in" parts (then again, this SSE business is much more of a CA DOJ issue than an ATF issue.) Along with the 07 discussion there is some additional discussion if added markings on the firearm are needed for said conversions. Current consensus is the FFL doing the SSE does not need to hold an 07 provided all the parts are "drop in."
There have been thousands of SSE conversions & transfers done in the last several years all across CA; a good friend who is an FFL has done dozens of them. He's even had an audit occur at the time he had several SSE's sitting in his possession during the 10 day waiting period. The auditor took a good look at them but acknowledged they were legal.
It's no secret the CA DOJ doesn't "approve" of SSE transfers as a means of getting around the safety roster... but if they could easily prosecute the FFLs and purchaser as you were advised I think we'd be reading about a LOT of arrests and legal proceedings from FFLs and members on Calguns. The fact that the SSEs are still continuing after several years seems to indicate that while the DOJ might not "approve" of them there is nothing they can do to halt the SSE process at this time. If AB-169 passes however the exemption will come to and end.
What exactly does your DA employer say they could prosecute an end-user/owner under if the pistol has been converted from single shot / SSE to semi automatic after the DROS? As long as the pistol is still in a legal configuration in CA following the conversion (i.e. no threaded barrel, etc) what specific PC(s) would apply to both the involved FFLs and the purchaser?
