1Password does not have a list of "secrets" that it draws from for all forms, it only saves the information you provide per-site; anything it submits to a site is information you've already submitted to the site previously.
1Password's extension does not prompt for my master passphrase, I have to click on it to enter it (if I haven't already). It also to don't try to fill forms on page load, I have to instruct it to do so. By default it will usually submit a form upon fill but I often turn off that setting.
As for browser vulnerabilities, I'm not familiar with any information about extensions being particularly vulnerable to browser exploits, it seems like when browsers get "pwned," anything in userland (if not the whole system) is up for grabs so avoiding the password manager's browser extension doesn't gain you anything. I'm not saying there's no risk, just that trade-off is worth it. Agilebits argues that using the extension is safer because it avoids keystroke loggers and clipboard sniffers [0]
[0] https://blog.agilebits.com/2014/08/21/watch-what-you-type-1p...