Yeah, a bit of an xy problem, I’ll admit. My ultimate goal would be invisible blocking/hiding from certain expected behaviors like the ones anti-adblockers employ. I’m not sure if what I have in mind specifically exists or not, but I find it fun to tweak and experiment with these kind of things. So I don’t necessarily have a problem I’d like to solve, it’s more an ask for directions/experiences.

I have tried Fiddler before, and it almost has the capabilities I’m looking for (it replaces an entire file; nothing granular), but it was a bit of a hassle to get working well last time I tried it, with the CA certificate, decrypting the TLS and stuff.

I haven’t heard of ZAP before, so this is new and intriguing! Thanks!

uBlock Origin I very much like, and I think it’s near-impossible for me to use a browser without it installed anymore. But as far as I know, it’s only used for blocking (dropping network requests) and hiding (CSS-like rules), but no manipulation?

The monkeys I’ve also used a little bit of, but I wasn’t aware that they supported being run ahead of the rest of the page like how I wanted, so this is definitely worth looking into, I appreciate it!
For reference, I found this answer on StackOverflow, which is promising.

I most likely am going a bit to the obscure/convoluted solution, yes.

My goal is to do something to the effect of uBlock Origin, but instead of just blocking/hiding, either replace with local files, or intercept req/res in order to manipulate them favorably, without being detected. I don’t know what uBlock does under the hood though, apart from its resource blocking and CSS-derived hiding.

Example:
Watching a video on youtube, an ad is about to get loaded, but instead of the hiding/blocking strategy uBlock uses, intercept the GET/POST, save the important flags that are uniquely served to your device that would indicate that you have successfully been served the ad, drop the rest, and then answer with what would be a valid response for “I have watched the ad in its entirety”.
So the server basically saying “Here, I give you this page and this script with both vital and ad contents. I now expect you to provide the corresponding hash that these two files will create through a series of functions. If you don’t, I will assume you’re blocking me, and I won’t provide further contents.”, and I’ll simply respond with “Here’s your hash! *wink*”.

Essentially, I wish to experiment with trying to be completely invisible in the blocking, by providing responses as if I have loaded and watched the ad, with all anti-adblock implementations through scripts and dynamic loading “intact” and unaware.

You’re quite right, I should include my goal in the post - editing it in now.