Unreasonable Boss

That sounds like a horrible work environment. I have been in one such environment and it's really soul destroying.

I could be normal that they reduce your hours, with appropriate notice, if that is what the company requires, but there are ways of handling that nicely too.

I've often worked with the assumption that my emails can be accessed but with the understanding they wouldn't be without just cause (like if a complaint was raised by an outside party, or other valid concerns that needed to be investigated). Otherwise I had my privacy and trust.

Normal? I hope not! Reasonable. No.

Work emails of course can be accessed by management.
That is normal and seems perfectly reasonable to me.
As for other issues regarding employment law perhaps you could look at ACAS website.
I presume you are not in a union.

It's very difficult when you're working for a small company as the owner is more personally invested with no experience of managing a company.

But this sounds like their personality. They seem quite insecure and controlling. I had a manager like yours and it was soul destroying, effected my morale and I had to leave.

For the website you can only pass on customer comments , but ultimately its his choice what to do. He can monitor emails, it seems an inefficient use of his time though.

Working from home doesn't mean he can speak to you in a bad way,

I'm not sure about changing hours as your contract is changing. I'd speak to Acas about that as if you've been there for more than 2 years I think you have rights, but check this.

But do you want to work for someone like this

If you are talking about business emails, your boss has the right to view them, because they are business ones.
They should only be for business use anyway, so there should be nothing untoward on them.

I had All staff access to everything, when I was IT Co-ordinator in my school. Even the HT didn't have that access. However, I would never have dreamt of going into anyone's account, without their permission.