Considering buying a Commando after a gap of six years

1.  Is the 850 more reliable and if so which Mk should I look for?

Going by past reports yes, I have an 850 MK2a and owned it for 35 years and never not got home, stuck in 4th due to layshaft bearing failure was the worst and cost me some blue clutch plates which are still working to this day.

2.  Has the problem with ethanol been solved in respect of carburettor tuning and component life?

Amal Premiers cover some but not all, at least the worst left ie the blocked pilot jet is now easy to unblock. Anodised slides have killed the premature body wear.

3.  Has a source of good new interstate and fastback tanks emerged?

Debatable, the Indian sources are variable, if you stick to the long serving ones like Royal Choppers and pummy8081 you stand a better chance but there will still be work to do. EMGO Roadsters are the best repro's but limited range. I run Interpol tanks as my solution. 

4.  If I can maintain Japanese machines and also given that I maintained a T140 by myself, is it reasonable to expect that I could maintain a Commando by myself, with occasional advice from other NOC members?

Depends, nothing Japanese will prepare you, the T40 should. 

5.  What is the best way to buy a Commando?  I am very wary of ebay bikes. 

Known long term owner who knows a lot about his Commando so use the sales lists on NOC. 

6. Is there much difference in terms of performance and reliability between the Atlas and the Commando?

Never had an Atlas but there is a lot more attached to a permanently vibrating engine.   

7. in relation to the 850 F1 series frame numbers, how much of a problem is it if the Machine is dvla registered with that number but still has a VIN plate on the frame with a 300 series number matching the engine and gearbox?

Mine is such an example, no issues at all, the VIN plate is there to see as is the F number and the F number is the frame number on the V5 and the 3xxxxx number is the engine number.

Depends on condition and the market, plus what the Covid is going to do to the market is anyone's guess.

Old Bike Mart is a better source of bikes for sale than Ebay, IMVHO.  The subscription is worthwhile and you get some good reading material too.