I live in Germany an every two years a car has to be checked officially if still acceptable to drive publicly. This is rather though. No oil drops only oily mist, no obvious rust holes threatening structual integrity or pedestrians etc.. Beside a hole in the rear left wheel arch that has been overlooked by all parties having had a scrutinizing look, another problem is oil leaking. Of course, all remnants are removed before official inspection. Nevertheless, the garage recommended addressing the causes. Since replacement of the rear main seal - one usual culprit - requires a lot of effort and might be short lived with respect to reappearance I postponed this task as well as a oil leakage at the front of the engine ... sort of. My temoprary (or rather infinite) bubba-style fixes: I put a fitting plug in a hole in the central subframe addressing the latter issue, i.e. oil is collected in the plug and at least not dropping on the floor. Regarding the rear main seal I put some heat resitant foam between the oilpan and the torque converter cover. I haven't tried it running longer trips, because if this has got some drawbacks I have not considered. Thus, I'd like to get your opinion.
On the oil leaks I would recommend a proper diagnoses before you spend a lot of money on front and rear crank seals. Oil leaks always settle near the bottom of the engine and especially the rear because of gravity and the wind as you drive down the road. So don't let a mechanic with just a quick look sell you on a big expensive job. I recommend fixing the leaks but understand if they are barely leaking but you have to deal with a rigid inspection. As far as the foam I would just make sure it is not very flammable . I know the possibility is remote but if it ever went up in flames you would never forgive yourself. By your description I am not following what you did with your frame. I am usually not a big proponent of snake oil in a can type products but in the past have used a couple with good results. I am not sure if this is available to you in Germany but I had good luck with this in both stopping engine oil and power steering rack leaks with no long term ill effects It's called at205 reseal. Here are some videos about it https://www.bing.com/videos/search?q=at+205+reseal&qpvt=at+205+reseal&FORM=VDRE Good luck with whatever you do as I know the stress of state inspections all to well.
I meant the part of the sub-frame between the 2 cross members, where the car jack can be placed to lift the front. The hole is infront of this lifting point. The pic shows the plug installed.
