[RPG] Can non-wizards decipher a wizard’s spellbook

I would say no to both copying and casting Wizard spells above level 1, based on the spellcasting rules under multiclassing on pg 164 of the PHB:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class

I read that as saying that you would count as a level 1 wizard, which comes into play under the and can prepare section. A level 1 wizard cannot prepare spells higher than first level. Nor would they be able to scribe them, as spells you are allowed to know are also based on individual single class.

PHB Example with wizard on pg 164 as well:

As a 3rd level wizard, you know three wizard cantrips, and your spellbook contains ten wizards spells, two of which can be 2nd level

The first errata to the PHB clarifies this explicitly:

Your Spellbook (p. 114). The spells copied into a spellbook must be of a spell level the wizard can prepare.

But, you would be able to cast a level 1 wizard spell as a 9th level spell slot, per PHB rules further below:

If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower level spells. If a lower level spell that you cast, like burning hands, has an enhanced effect when cast using a higher-level slot, you can use the enhanced effect, even though you don't have any spells of that higher level.

Yes, but only 1st-level spells

PurpleVermont has already argued why such a multiclass could only write down spells of a level they could cast as a Wizard (so actually up to 2nd level, if they take Wizard 3). I disagree with Vermont's argument that stolen spells could not be transcribed at all, however.

The rules don't clearly state that you have to have seen a spell written down to be able to transcribe it, that's a matter of interpretation--and I disagree with the interpretation:

Copying a spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation.

The fact that different wizards use different notation implies that the written form of the spell is not an inherent part of the magic, merely an aide-memoire. And the requirement to practice the spell until you understand its V/S requirements -- precisely the thing which the Arcane Trickster can instantly intuit -- strongly suggests that this is the most important part of "learning" a spell, and the rest is merely paperwork that your first-level Wizard training would absolutely prepare you for.