I've noticed that some novels don't list all of the available chapters. e.g. God and Devil World there are links for up to 539 on the novel page despite there being more chapters available. I think the rest were not added due to them being posted on webnovel (which will likely have them behind a paywall) but there are some on the original translators site too (translationnations [dot] com) Can we get the list of chapters updated to show that they are available?
NU side doesn't really have a direct communication with translators. It's best you ask the readers for said list than asking NU for it. If it's available to the public on the translator's website, you can of course submit the releases so it will be listed on NU. However, unless it's submitted, then we don't know about it
https://www.novelupdates.com/profile/ Your NU Profile has the options to add releases. Do understand that they'll be added to a queue requiring a staff to manually approve them, so it would probably take a while before they show up on the NU page.
Related question... in the case where the chapters are not added, is there any way to track our reading progress besides using the Notes page?
You can have them set in a separate reading list that uses the manual reading list. However, that means you have to manually note the chapter number.