3) He's an idiot who hasn't given it a shred of thought, and was hired for loyalty, not brains.

I try to apply Hanlon's Razor to this administration, but it's hard not to occasionally entertain other explanations with the sheer volume of incompetence.

I think that as someone's power and authority grows, the risks of applying Hanlon's Razor get too high. It's best applied to peers.

His power and authority prior to getting put in charge of the largest military in the world was being a talking head making bad calls on television, which is roughly a step above being a 'social media influencer'.

The same reason teenagers might use Instagram DMs to communicate about school projects - It's just the platform he's familiar with.

Or the same reason I have Whatsapp - communication in my social groups happens there, and if I don't have it I get left out.

Your explanations assume there is some deeper meaning, looking at the tradeoffs for each communication platform, and then coming to some rational conclusion. I don't think there's much evidence for that.

The people around trump just happen to be used to using signal to communicate, and if Pete doesn't get on board he gets left out.

Incompetence only applies if you're not running the literal federal government.

We have to assume malicious intent. These people could start a nuclear war. They get zero flexibility or grace.

Like with a cloth right?