Reddit mentions of eSynic 3 Port Toslink Switch Digital Optical Audio Switcher 3X1 with IR Remote Control Aluminum Alloy SPDIF Switcher 3 In 1 Out with 6.6ft Optical Cable Supports PCM2.0 5.1CH DTS AC3

ESYNIC 3 Port Toslink Switch... https://www.amazon.com/dp/B074W8DG3P?ref=ppx_pop_mob_ap_share

E10 has an SPDIF input correct? Something like this: https://www.amazon.com/eSynic-Switcher-Aluminum-Material-Dolby-AC3/dp/B074W8DG3P/ref=sr_1_3?ie=UTF8&qid=1550760148&sr=8-3&keywords=TOSlink+switch would allow you to run a tos cables from the PS4, PC and laptop (you could use hdmi for that) to the e10 and use the switch to move from one to the other.

To get MacBook air to optical out, you can use a USB to optical converter like this DAC (can't vouch for quality but reviews are good)
Signstek HIFI USB to Coaxial S/PDIF Converter Convert Digital to Analogue Signal Mini USB DAC PCM https://www.amazon.com/dp/B00FEDHHKE/ref=cm_sw_r_cp_apa_E2maCb7NXB9B8

For your second question, you could use an optical audio switch. Then you can use the remote to switch the inputs.
ESYNIC 3 Port Toslink Switch Digital Optical Audio Switcher 3X1 with IR Remote Control Aluminum Alloy SPDIF Switcher 3 In 1 Out with 6.6ft Optical Cable Supports PCM2.0 5.1CH DTS Dolby-AC3 https://www.amazon.com/dp/B074W8DG3P/ref=cm_sw_r_cp_apa_F.maCbYFW5QB2

It's working great so far, but there are a few quirks. I have to manually toggle the optical input with a remote to switch between audio for my PC and PS4, but i'm not the type of person to have videos streaming while i play games. If i really wanted to, i could plug a cord into the AUX on my monitor and my Logitech 2.1 speakers to have concurrent audio.

Here's my setup:
PS4 HDMI is hooked into this box to split the audio out of the hdmi
HDMI from that box connects to my monitor and the optical cable plugs into this optical audio switcher
One optical cable goes from that switcher to my PC and the other connects to the speakers here

I haven't noticed any signal degradation despite the double boxes and extra cables stretching into every direction.

The DAC in a receiver may or may not be any better than the DAC in a Chromecast Audio. The DAC in the NAD is probably better than the one in the TV. Could you hear a difference, maybe, maybe not.

One option is to convert analog to digital with a converter and then use an optical switch for 3 optical inputs or use an HDMI Chromecast on your TV so you only have one optical signal going into the NAD for those instead of two. However, converting the turntable signal to digital defeats much of the appeal of analog vinyl though you may or may not notice a big difference.

If you are going to upgrade, skip getting another slim amp which often have limitations and upgrade to a full size receiver.

Receivers:

If you are using it with a TV, I recommend an AV receiver for the HDMI switching even for just 2 speakers, since they have a high pass crossover for your speakers if using with a subwoofer, room correction and setup EQ with the including microphone and the option to add a center for better dialog and side speakers for surround sound. These both have dual optical inputs, 4 HDMI inputs and an internal phono stage phono input. Refurbished with a one year manufacturer warranty.

• DENON AVR-S640H 5.2 w/HEOS, $379.99 $199.99, 47% off msrp, Audyssey Multi EQ.
  
• DENON AVR-X1500H 7.2 w/HEOS $599.99 $299.00 50% off msrp, adds Audyssey Multi EQ XT and the option for rear or Atmos channels in addition to the side surround channels.
  
  Or a stereo receiver with 2 optical inputs and phono input but less features:
  
  
• ONKYOTX-8140 $399.99 $199.99
  
• Or the ONKYOTX-8270 $599.99 $349.99, is one of a few stereo recivers with HDMI switching along with the INTEGRA DTM-7 $649.99 $419.99.
  
  Turntable:
  
  The LP120 is ok but it is plasticy and some owners end up rewiring or ripping out the not so great internal phono stage/preamp and bypass switch that does not completely bypass it in order to use a better external phono stage/preamp.
  
  Consider looking for a vintage 70s direct drive. The LP120 is a copy of the much better Technics 1200 and there are other Technics models that have lower prices since they are not in demand from DJs like the 1200 is.
  
  If buying new, I recommend the Fluance RT82 with Ortofon OM 10 $300:
  
  
• Belt drive for motor noise isolation.
  
• Auto Stop
  
• Optical sensor speed controlled motor for lower wow and flutter and speed variation specs than the LP120.
  
• Solid wood/MDF plinth, available in gloss black or walnut finish.
  
• You can also later upgrade to an acrylic platter that Fluance plans to offer separately soon for around $90. You can see it on the $500 RT85 that has the same motor, just different cartridge and platter.
  
• Optional upgrade now or later to an external phono stage/preamp with the Fluance PA10 $80 or the popular Schitt Mani on sale B-Stock $110 shipped.
  
  (Skip the cheaper RT80 or RT81 that do not have the optical sensor speed controlled motor.)
  
  Subwoofer:
  
  If you don't have a subwoofer, have you considered adding one to go with the KEFS?
  
  
• Lowest budget subwoofer: Dayton Audio SUB-1200 12" $150.
  
• KEF Q400 Q Series 10" $699.99 $399.99.
  
  Helpful Guides:
  
  Home Audio: Introduction to Audio Components | Lifewire • Intro to home stereo systems • Zeos Tutorials, Diagrams and Videos • Guide to Home Audio • AverageJoeAudiophile's Guides
  
  Home Theater: r/HTBuyingGuides FAQ • How to Set Up a Basic Home Theater System - Lifewire • How to Set Up Your Home Theater Receiver • r/hometheater.
  
  Speaker Positioning: Speaker Placement for Stereo Music Listening • How to position your speakers for great sound • Speaker Placement for Home Theater.
  
  TV Positioning: Vertically centered at eye level while seated.