The best historical source is Hunminjeongeum Haerye.
The document was published only a few years after introduction of the script and thus there's high likelihood that it gives truthful account.
Of course it's also possible for the document to gloss over some bits and emphasise cultural norms at the time over practical reasons for vowel glyphs. That is document could have been created to prove worthiness of the script rather than as a historical account.
Thus, I would view yin/yang vowels as artefact rather than deliberate design feature.
Likewise, even if consonants correspond to the shape of tongue and mouth (why these two? why for those specific consonants? why head looking left?) it may or may not be the reason that consonants were created like this.
Whether glyphs were designed according to current understanding or got their shapes accidentally (or the mix thereof), they creators were certainly at least inspired by other writing systems.
Where did the inspiration come from?
- Chinese? (probably not, result would be like katakana or manyogana)
- traditional Mongolian? (IMO only minor similarities)
- Phags-pa? (has identical glyphs, but different meaning)
- Devangari? (about as likely as the above)
- Tibetan? (-"-)
If the tale of design by committee is true, surely the scholars had knowledge about these scripts and texts for most of these (Buddhist monks travelled a lot, and had scrolls).