Welcome to ForFour ownership!
To address your questions directly:
1. That is most likely the clutch actuator you can hear. The cars automatic gearbox is not a traditional auto with a fluid and a torque converter but is essentially a manual with a automatic clutch, which uses a mechanical actuator to release/engage the clutch. This is called a AMT (automated manual transmission). The noise you are hearing is most likely that.
2. Sounds like it could be the middle section of the exhaust. I'd advise jacking the car up anyway (there is a suitable jack in the boot, under the floor).
3. Given the list stated already and the mileage, I'd possibly consider changing the spark plugs as well. Does the last documented service show this as an item? Smarts are traditionally serviced by letters, so A (a minor service) and B (a major service) so from the description, it sounds like a A service is done.
I'd also note that the car as a creep function, which you've probably noticed. For me, the creep function is fine but it makes parallel parking a bit of a faff because it seems to go very quickly in 1st or Reverse and requires a bit of brake work to not collide with my neighbours cars.
As a 1.3 AMT owner, I can advise that using A around town is "okkkk" but it is fairly dim witted and much, much,
much better on the motorway. Kickdown is fantastically addictive and once you sail past 3400rpm, the valve timing changes and becomes more alive sounding. Conversely, around town, I can toddle along in 6th gear at 30mph whilst in manual mode, very quiet.
I'd also advise leaving the car in gear when stationary at lights (put the handbrake on, lest ye fall foul of the creep) and in R when switched off.
One final thing - as noted by
CrazyG, the AMT gearbox + clutch actuator in the ForFour needs to be retaught every year (so every major service) which can only be achieved with a STAR machine. S2Smarts in Watford are around your way (though I confess, I don't know exactly how far away that is for you but it is worth the trip) so it may be worth taking it there for that.
The engine itself is fanatically reliable but the actuator can be problematic though given the mileage, I suspect you'll be OK for a while yet. To increase your chances of not having actuator failure, when you do get the actuator retaught, I'd also ask S2Smarts to lubricate the actuator as well but I don't believe this is a yearly thing (once every 2 years maybe?). Otherwise, the car itself is simple enough to work on.