First up this is not a grey area at all. Requirements for coil overs are pretty clear in this document from DTEI. You can start your mods now and get an engineer to look over it once done. But do it properly.
it's important to understand why it's expensive to engineer modifications. I'm a mech engineer and have considered getting my CPE, in particular so that I can approve coilover installs as it's fairly straightforward to do, a few hours spent to look over the car and produce a report. Keep in mind that at a fairly standard round 100 bucks an hour, you wouldn't get much change from 500.
The main reason why I haven't done it: professional indemnity. If I approve a suspension modification and after a crash a coroner rules that suspension failure contributed to someones death, I am personally liable and could lose everything I own. Professional indemnity insurance is available but wont do enough of this work on the side to pay the insurance bill.
Anyway, the biggest issue is getting round the heating issue, you'll need to get any welds x-rayed if you use weld on coilovers.