Does anyone know what the pitch of the underbody Torx screws that hold the wheel arch liner and engine tray in place is? eBay has 4.8x15mm or 5.0x15mm options.

Do you mean these ones:

 

https://www.ebay.co.uk/itm/Genuine-VW-Audi-10-Wheel-Arch-Undertray-Interior-Exterior-M5x16-Screws-N90974701-/173252476525

Well yes, they look like that, but there are two options on eBay and I can't recall which ones are the ones.

 

https://www.ebay.co.uk/itm/VW-SELF-TAPPER-SELF-TAPPING-TORX-SCREW-ENGINE-UNDERTRAY-SPLASHGUARD-4-8-x-15-mm/171102498299?hash=item27d680edfb:g:jE8AAMXQPW5SFhVn

or

https://www.ebay.co.uk/itm/VW-GOLF-GRILLE-BUMPER-SKIRT-TORX-SCREW-BOLT-WHEEL-ARCH-SPLASHGUARD-5-x-16-mm/190997921205?fits=Car+Make%3AVW&hash=item2c785cfdb5:g:LLcAAOxyNo9Spe9n

Well my link is equivalent to your second link which I'm sure is the correct one - the only difference being mine is the genuine part. VW Part no. N90974701 to be exact. I bought some myself from a Skoda dealer to replace some missing undertray ones on my 2017 2.0L Hatch a few months back.  If it helps you can verify that its the correct part no. with the aid of this site:

 

https://skoda.catalogs-parts.com/#{client:1;page:group;lang:en;region:cz;model:octavia;modification:oct;year:2016;modetype:753}

Nice one, thanks.