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"What would happen if I mixed X and Y when X was supposedly only supposed to go with Z?"

Well, that's what you're MEANT to do, mix things up and find YOUR style :-)

However:

Sounds like you're OK on rudiments. You read the language. Now you look at songs.

Study the 'Golden Age' American composers - Kern, Gershwin, Porter, Berlin. You shouldn't have too much problem with harmonic analysis - they're pretty well based in Common Practice. A bit of Blues creeps in, and they can be (over)fond of the Chromatic Mediant (ii, V, I in C followed by ii, V, I in Eb got to be a bit of a cliche). But they don't go in for much that's too off-the-wall. But look at the craft. The rhyme-schemes in the lyrics. The fit of music to lyric - "But how strange the change from major to minor" (and if you don't recognise that quote, I question your interest in Popular Song!)

Then, in the 50s and 60s the prevailing style simplified. The 'Three chord trick' (well, actually it was often a 'Four chord trick' - C, Am, F, G7).

Then Bacharach and The Beatles. They shared a liking for the ♭VII chord - something that still unsettles a lot of users here who have been brainwashed into the idea that anything non-diatonic MUST imply a change of key!

Then the reaction against traditional melody and harmony. The Top 100 filled up with chanted lyrics over a repetitive, looped 'beat'. And the guitar-strummers, stringing chords together in ways that defy structured harmonic analysis.

And more. But that'll do for now! Is there anything above that is alien to you? Anything that you couldn't look at the sheet music and recognise what I'm talking about? Anything that makes you say 'Yes! THAT's the sort of song I want to write!'? Answer those questions, we may have some specific advice. But, given that you're already musically literate, I'm afraid the advice will very likely be a list of songs to study, dissect and try to copy. Precisely what you DIDN'T ask for. Sorry!

Songwriting is a lot different from knowing theory.

Knowing a lot of theory won't necessarily make you a good (or any sort of) songwriter.

There have been many, many people who have little or no theory, but have written some brilliant songs. There have been many, many people who have vast theory knowledge who have written some awful songs.

With grade V theory under your belt, it's more than enough to use to write songs (theoretically!), but there's a heck of a lot more to it than knowing that a B♭ chord works well after an F7. And putting X with Y or Z is part of the journey. If every songwriter kept to 'the rules' (whatever they might be) we'd still be driving cars with square wheels, if that makes sense!

And there's always the 'read all about it, therefore I can now do it' approach. It often isn't true. Have a go at it, and after a while it'll get easier is maybe a better way to regard things.

Do you sit at the piano and doodle? What comes out? Sometimes it may be good - others mediocre. But at least record it, either in sound or on paper.

Listen to a lot of songs in a style you'd like to emulate. Do what the writer of 'How to become a Good Songwriter' would do. Analyse, swap bits round, change a few notes/chords. Throw a bit of theory in if you must - the chord usually contains the main note/s sung, etc.

Above all, remember - you have to kiss a lot of frogs. A bit like shooting a film - ten days work for ten minutes in the can - if you're lucky. Sorry, can't recommend any books - mostly 'cos it isn't allowed on this site!

If you were to teach me all the music theory in the world for songwriting then what textbooks would you use?

Taking the question literally - all of them!

Of course the problem with that is that reading all the books won't leave you any time for writing songs...

Even answering your question a bit more sensibly - I'm not aware of any single theory book that covers a large percentage of what you could usefully know as a songwriter. Most theory books are written from a specific perspective, either in terms of level, or style, or degree of abstraction.

As a couple of random recommendations on the music side...

What makes music sound good?
Music Theory through Performance and Composition

But I think the harder thing in song writing is the lyrics!

Without any idea of what tools there are, then what tools can I use in the first place?

There's an almost infinite number of tools you could use. Again, the problem is that if you spend your whole life learning about all the possible perspectives you could take on music, it doesn't leave much time for actually making any.

This is why the approach of looking at particular songs is a good one, because it provides more focus - you can take that particular song, and ask yourself whether you have the ability to make sense of how and why it works. If not - that's your cue to learn a bit more theory.

I've looked at a lot of answers similar to this question and it always goes along the lines of.... "Don't study music theory because it inhibits creativity & expression!"

Well, knowing theory won't limit creativity - but taking the approach that you can only do things in music that are 'allowed' by some rule or other might limit you. On the other hand, if you're not finding any inspiration from elsewhere, following some 'rules' could be inspiring - and of course 'rules' are arguably what define existing genres.