Re: How much access does google have to your N5?
Well say there's someone who really doesn't like you high up in google and he had the authority to access this sort of stuff. Would he literally be able to just log into your phone like an email and browse around it. Read all your texts, send out fake ones, steal stuff off your phone, generally screw around with it.
Google have to follow the law so that means complying with the NSA and other federal agencies, so all bets are off, doesn't matter if it's your N5 or what you typed in Google search.
Now with that aside, just check what Google services are capable of doing to your device and make the deduction yourself. You can go into Settings -> Apps and see the permissions allowed for each Google apps and services. You can remotely track and wipe your data with Android Device Manager, remotely wipe Wallet, sync files. Google lets you know when your phone is syncing or connected with Google server, 4.3 and before, if the Data or WiFi bars turns to blue, its on. For KitKat, its in the QuickToggles menu, with the bars turning from orange to white. It's up to you to make a conclusion.
If you are that concerned about using Google, unlock the bootloader, wipe the phone and install another operating system on it. Or use an AOSP rom with no Google apps installed.
Before being so concerned about Google, look up Microsoft and Apple, they have their fair share of abuses and concerns for as well.
One of the most hilarious thing I've ever seen was some years back when tech news sites started reporting of Google saving search resutls for a long time, and there was a mass outcry for the internet masses. So the same tech sites started creating articles recommending how to protect privacies and giving alternatives to Google search, and one of the recommendation was to use Bing search instead since it only save search data for half a year, so droves of people switched to Bing feeling more secured and went on their merry way.