You can load ebooks from any source even on a black listed Kindle.
Ads are not in books and not on all kindles, only if subsidised and then you can contact Amazon. Sometimes they turn them off for free and other times charge.
The listing has:
So these are subsidized by Amazon.
I've only ever seen ads on the home screen (not on Library view) and sleep screen (not seen much if you have a sleep cover).
I don't know if a jailbreak blocks adverts.
Note, the only limitation on books via USB is that azw3 is best. Calibre will convert any format without DRM.
Amazon "Send to Kindle" delivers any epub without DRM by WiFi.
The main reason to Jailbreak would be to use KOReader.
This model includes a Mobile/Cell modem (2G/Edge/3G/4G) which may or may not work depending on your location. That really only works for content sent to your registered Kindle from Amazon.
P.S. Woot! has been owned by Amazon for over 10 years. Hence Advert subsidy. The models with cell/mobile are more expensive for the same thing and as it's possible to use a phone/tablet as WiFi hotspot or to copy downloaded from Amazon via phone USB USB2Go mode, there is less demand. Latest models don't even offer the option. So this is actually Amazon selling off NOS. Some might be returns found to have no faults (sometimes people realise they don't need cell/mobile, return in "no quibble return window" and get the WiFi only version).
Amazon using Woot! may reduce support/warranty costs and avoids much effect on current model sales. Amazon used to just offer NOS on their main site.