Electrical Question: splicing feeders and reducing size to fit sub panel lugs

Hey everyone. Thanks to anyone who has the time to help me out. I will explain my situation and then ask follow up questions.

I have property with a 600' driveway leading to a cabin. There will also be a detached garage about 250' into the driveway, and perhaps in the future another cabin or house near the garage.

I had an electrician help design and install the electrical service to the cabin. The meter/main shutoff has a 200 amp breaker. From here, we ran ran 3.5" sch80 Conduit underground, with large pull boxes near all of the future buildings. We have 250kcmil Al (2 hots 1 neutral) and 2 AWG Al ground running all the way to the cabin, with enough coiled in the boxes to make splices.

The cabin has been up and running for about a year now, and I am currently building the garage with small apartment above on my own. The electrician, who is no longer involved, instructed me to use polaris taps in the pull box outside the garage to splice the 250 feeders and ground over to the garage subpanel. So far so good, right?

Now, I was planning on basically mimicking the panel work that the electrician completed at the cabin, but after doing a lot of research, I am wondering if I need to take a different approach and perhaps have a problem I now need to address at the cabin.

Inside the cabin, the electrician pulled the 250 feeders and ground into a trough below the panel, and used polaris taps to reduce the 250 Al to what I am guessing is 2/0 Copper (there are no markings on the wire, but that's the largest size that fits into the Square D 100 amp panel).

Now for my concerns and questions:
1) Is it ok to reduce the feeders like this to fit into the 100 amp panel? It was my understanding that each feeder was carrying the 200 amps and the wire ampacity must be able to handle this. I've seen some people argue this short length of wire is protected by the 100 amp subpanel, but that doesn't necessarily compute for me.

2)It was suggested to me that if I install a 200 amp subpanel in the garage (although this is way bigger than needed), the panel should accommodate the larger feeders, and then I do not have to worry about reducing or any of the issues that may come with that. That seems simple enough for me, but do I have a big problem on my hands at the already installed and live cabin sub panel?

3) As stated, each subpanel is fed by 4 wires and grounded all the way back to the meter/main grounding rod. I see nothing on the cabin subpanel in terms of a grounding electrode. Is this an issue as well? Before pouring the footer for the garage, I asked the electrician if I should be running anything for grounding (this was when he was still involved; he has since moved). He said something along the lines of "no, it will be taken care of later at the meter". Is this another thing I should be worried about?

I like to have all the information before calling anyone out, as I know different circumstances lead to different methods of install and whatnot.

Most of all, I don't want to be cheating off this electrician's paper if he was taking any shortcuts.

I appreciate any advice.

Thanks!