Good day all,

I bought my 2004 RX8 147KW with 106 000 km on the clock in April. I live in northern Germany. Before I bought it I've had them do a compression test and the results came back fine, no less than 7.0 (done at official Mazda Vendor here in Munster). I was told the car was fully serviced, new spark plugs, filters etc.
I've bought the car not as daily commute but for long trips, holidays.
I've hesitated for a while before making this post having spent a lot of time browsing the threads here, but none of the threads I've read exactly matched what I'm experiencing. The problem started in August:
I've noticed that it took a bit longer for the car to start, hot or cold. I was spinning but it wasn't igniting. The problem got progressively worse, and there really was no difference between a hot or a cold start, both took about 3-5 seconds. Then I had to drive all the way to Estonia, about 2300km, (before the trip I accidentally overfilled my oil by a fairly considerable amount) and on the way it refused to start at the gas station. It happened once, and then I got it going again and eventually reached my destination.
So I thought: spark plugs.
Finding a mechanic to service my RX8 was not easy. Nobody wanted to touch it.. Eventually I found a guy, he changed the plugs, said two of them were okay-ish, two were quite worn, all four were wet though. He also said when he put the new plugs in it started like new. But then the problem came back again.
Next I checked the air filter and it looked like it was NEVER changed. It wasn't oily, it was just clogged with all sorts of stuff. So I put in the new filter, had the airway and the MAF cleaned. It had a positive effect, but the problem didn't quite go away. Still took 2-4 seconds to start. The guys who cleaned the air way (at a non official Mazda Help centre) said they suspect that I'm running rich, because of the way it smells. And that that could be causing the slow start. They didn't know how to solve this issue or how it could have come about..
So then I had to drive all the way back to Germany and after about 3/4 of the way I see CEL. Checked it through OBD2 and it came up with u0073 ( Control Module Communication Bus 'A' Off) & p0420 (Catalyst System Efficiency Below Threshold (Bank 1)) codes.

I gather that u0073 can simply come up just by plugging in the OBD2 scanner, so fine..
p0420 is, I assume, a consequence of having had starting issues for a while.

The car drives fine, no loss of power. Normal oil consumption, mileage is only slightly above normal (16 L/100km town; 11L/100km highway). I do not premix.
So my questions are:
1. I get no misfires, so am I safe in assuming that the problem can't be caused by a bad ignition (plugs, wires, coils)?
2. I've read about the Coolant seal failure, and I have noticed white smoke on startups on a few occasions, but it's been very humid here so I can't be sure it's not condensation. Can burning coolant clog the CAT?
(Another thing that points to the coolant is this one time I was driving in the rain and a puff of white smoke came from under the hood. It didn't last long, but it was noticeable. On visual inspection I've noticed no leaks though..)
3. Running rich. I don't even know where to start here. Can a bad air filter that's not been changed for too long a time make the computer go permanently insane and give me a bad mix? How can this be fixed? (The car does smell a lot of petrol behind closed doors for the first few minutes after I start it, but I heard that's quite normal.)
4. Could I in theory have lost enough compression in 6 months and 7000km to begin having these issues?

Obviously, I want to solve the core of the problem before I begin dealing with the CAT.
Help me establish a logical chain of thought, please!

Arthur