I do not want contacts available across email accounts.

At home on my windows 8 desktop I have added my personal and work email accounts. Problem is I constantly send emails from the wrong account because all contacts are available from both and also there is apparently no way to change the appearance of the UI based on the account you are in. Serious shortcoming of the product IMO.

Any way to address these?

You don't mention what you are using for an installed E-Mail Client or if it's Webmail/Online E-Mail. If it's an installed E-Mail Client, which one? Does the program have a Change Identity feature so you can switch into a different account? If it's a Microsoft product have you looked in the Contacts Folder under your User name in File Explorer, Users? That may be where the contacts are getting merged together, may need to be put in Groups. If it's a Webmail/Online service such as provided by your ISP, Hotmail.com, Outlook.com, Yahoo, Gmail, etc., it may be in how your Contacts are stored, whether or not in groups, etc.

One possible solution is Mozilla Thunderbird. While you can set up separate accounts, 'cause I never have done so, I cannot confirm it's impossible to send email from the wrong account.