Can't arrange windows in 2016

a). That has been discontinued since Excel 2013. You could just hit ^ button at bottom right of ribbon and hide it. While each workbook opens in new window, it all shares same instance of the application. This was done to ensure all workbooks open in same instance (thus saving resource and making coding simpler by ensuring only single instance to the application runs under normal circumstance). This also allow freedom of having multiple windows and arrange as user sees fit.
b). That's not really function of Excel. If you want to customize window arrangement, use some window manager that suites your need. Google and you'll find plenty of free ones out there.

If you've been used to older Excel version, you may find it disconcerting at first. But I find new arrangement much easier to navigate using some custom tools once you become accustomed to it.

As @CK mentions:

A) This is intentional and has numerous benefits. It was done after years and years of users complaining about everything being restricted to a single Window in Excel. In XL2010 and prior 1 window = instance of Excel. An instance of Excel can see all the open workbooks and share data between them using MS Office built in feature like pasting special, moving sheets, etc. Being limited to multiple files in a single window was a huge hindrance as it made using files across multiple monitor displays a nightmare. It also made it easy to accidentally place a file outside the visible area of a window, causing many people to either not know they had a file open or to be unable to find it and use it, leading to lost work as most people didnt know how to get it back into the visible range of the window (Arrange All) and would instead close and open Excel to try and get it back.

As mentioned you can collapse and expand the ribbon. Now each window is in the same instance preventing issues with multiple monitors or arranging windows on screen. Each window stands on its own, meaning if you have 3 monitors, you can spread 3 windows across and each has its own ribbon so you dont have to go "all the way" back to another screen to use the ribbon for another window.

B) This is a Windows feature, not an Excel feature as far as I understand what you mention from your description. Windows 10 specifically has changed how Win+Tab and Alt+Tab work (presuming your on 10). They both used to essentially do the same thing with a different visual, now one is for moving between virtual desktops/workspaces, not open Windows. This is a feature Linux and OSX have had for at least 20 years and MS just decided to grace us with. Its a huge boost to productivity.

With Virtual Desktops or Workspaces, think of them as virtual screens. It goes beyond that however as it lets you better organize your workflow. Imagine if you will a small desk, with papers on it. As you add more papers, you eventually have to put some on top of others. At some point it means sifting through the papers on the desk to find stuff under the top layer of papers to find what you need. Not very productive. Virtual Desktops/workspaces is like putting a leaf in that desk to make it bigger (or simply having a bigger desk) in which case you no longer have to stack papers on top of each other but can spread them out. Now to find what you need you just pan (move your head) left/right until you see what you need. Alot easier right?!

Well virtual desktops is alot like that, cycle across one or 2 to get to what you want instead of shuffling between 30 open windows until you find what you want (and in turn shuffle all other open windows into a different order).

You could for example have a virtual desktop with all your browser windows/tabs open, on another all your Excel workbooks, on a 3rd your email, messaging, social media and on a 4th any work specific programs like SAP or a timesheet, etc. That could easily be dozens of windows that are now instead nicely organized between 4 virtual desktops and you cycle them with Win+tab (I think is it now) making it easier to move between tasks like email, XL, researching on the web and work specific programs.

Thats not to say there isnt a learning curve, you will have to change how you work. It will take getting used to. There are many new keyboard shortcuts to learn, many of the known ones have changed, and some of the staples are still the same.

Quick tip: start using Windows snapping feature if your not already. It lets you resize windows and snap them "together" to quickly place windows side by side, arranging them on screen to make comparison easier. Windows 10 lets you do 50/50 like older versions but even 25/25/50, 25/25/25/25, maybe more too. Its quick especially using the keyboard. Win+Arrow key will snap the active Window according to direction. After a couple of mins playing with it you will quickly see the power of snapping. Same thing can be done using the mouse by dragging a window by the titlebar to the screen edge or corner.