Good point, I understand many European and UK cities are now pursuing rules like what you describe. Probably justified in terms of public health, but the rules certainly will make it a hard argument to keep maintaining and operating a car like these.
Given that context, your plan of trying it out as it is for now and keeping a frequent eye on the belt does sound reasonable enough. It sounds like your second mechanic was able to take measures to keep the oil leak mostly at bay, so who knows, maybe you will get years out of it like this if it doesn't deteriorate any further. Oil contact in any amount is definitely not good for the belt, but a little oil mist is probably not as bad as being completely soaked. I think if you plan on popping the cover off every week or two and checking on it, you'll probably be safe enough for a while, assuming you are starting with a fresh belt. Watch for any sign of greasy or rubbery buildup on the teeth of the timing sprockets, or any signs of the belt swelling, softening, getting looser or tighter than it originally was, or changing in any other way to indicate it is being affected by the oil.
Of course also, if you ever start to notice evidence of any significant amount of liquid oil slinging out from behind the cover, or leaving drops on the ground, etc, in other words signs that the leak has suddenly increased, then you will need to take other action. For two reasons -- one, that it will mean the belt will have much more severe contamination and risk of breakage, and two, if the leak suddenly gets worse then that might mean the damaged threads for that bolt have let go. That would mean, in addition to allowing leakage, that the bolt is also no longer doing its job of retaining the timing belt idler pulley securely in the oil pump housing. If that idler pulley walks loose, then it will cause the timing belt to slip and valves and pistons to collide and you know what that means.
Good luck and let us know how it goes.
