Yes, adjust brakes as you say, but you need to repeat that from time to time as the right pad wears and the gap increases again. Don't squeeze the lever too hard though or the right pad will end up always rubbing against the rotor. It's a knack you soon get the hang of it.
It's only very recently Fiido started including mudguards as standard for most (all?) of their bikes; they used to be a £20 option. It appears to depend on who you bought your bike from as to how good the support is: Buying direct from Fiido seems to give the best prices and decent support.
I did nearly buy a D4S instead of a D11. Yes, you can easily remove the D11 battery for charging. That suits my purpose as I use the bike for short hops between trains - carrying the battery in one hand and the rest of the bike in the other spreads the weight nicely. The problem for some is you can't lock that battery to the bike, so you need to take it with you if leaving the bike somewhere - but since you're taking both the seat-post and saddle, it must deter thieves.
I honestly don't know the range of the D11. As some reviewers have said, it definitely runs better with a fully charged battery so I rarely let mine get below 1/2 full. And as I'm quite heavy and use it mostly in top assist, my mileage wouldn't be fair/typical. But my range is comparable to my other branded-cell 11Ah batteries. Given Fiido are now making and selling 400,000 bikes a year, we'd be reading complaints if their batteries didn't deliver or were failing!
I do wish Fiido would offer "official" rear racks though. I've had to order one from China that looks right for the bike, but finding one that was a simple light-weight alloy design that takes 25kg and fits (I hope!) was difficult. Decathlon do a cheap steel/10kg load one for their B'Twin e-folder that would fit, and there are full-pannier versions in Germany, but too much for my needs.
But who can complain? IMHO both D4S and D11 are excellent value for the money, and given the easy supply of spares, should both be on the road for a very long time.