I would also be interested in this for my bike as well.

I've tried both.  I prefer a milk crate that is cheaper and quick to attach/remove.

Let me know when you get it, also don't forget to puncture proof your tires before your first ride.

I also added a backrest to my bike.  I got it off Amazon.
https://www.radowners.com/index.php?topic=890.msg4472#msg4472

Rotate it a bit so the key can be inserted and removed easily.

I am in the Fremont east bay area, looking for others that would like to go on rides nearby.

If it is like the Rad Runner, you can cover 1 pin in the power connector to disable speed sensing and allowing a bit higher max speed.

I have the K Rads on my Rad Mini ST, I prefer them over the stock street tires the Mini came with.   I ride on pavement, dirt and packed gravel with no issues.  I would be careful riding in loose gravel with any tire.  The K Rads seem to be a good all around tire. Just make sure you put some flat protection in them before you ride(liners and sealant).

I have the same one as Wkindred but I got mine on Amazon.  Seems to work well.

1. I'm not sure what are top rated, I still use stock tires and tubes on my Rad bikes and they have held up fine after using liners and slime.  I use the K Rad tires that normally come on the Radrunner, on my Rad Mini ST.  I find the K Rad to be the happy medium between street and trail tire.

2. Not sure what Trekking involves but I would assume if you are comfortable on the Mini then it would be fine.  I have a suspension seat post in addition to the front suspension and it makes it pretty comfortable.  I also changed out the handlebars for riser bars so I could have more reach adjustment which adds comfort.

3. From what I've seen most mid drive do not have throttle unless you want to get a Baffang Brand.  There are some nice Baffang mid drive bikes but they cost significantly more than Rad hub drive bikes.

I did not try the extension with the stock bars so not sure.

Rad tires are notorious for being made out of gummy bears. Their "puncture resistant" tires are a joke and must be reinforced before riding to prevent flats.  I second the suggestion of using Mr Tuffy liners.  I use Mr Tuffy liners and slime and have not had a flat since they were used.

I replaced my Mini Step Thru handlebars with this:  https://amzn.to/3wq7QXW
and this:  https://amzn.to/35gipRa

The grips were a pain to swap over but I like the end results. 

I also put this on as well but it needed some shims.  https://amzn.to/3vmxU4C

What would cause a good battery to drain while unused till it goes bad?

I tossed them in the trash . Just kidding, I kept them as spares.