I'm familiar with the use of the caret symbol as a proofreader mark for inserting text, but thank you for Bill-splaining it to me. But as you allude, not only is the symbol going the wrong way for that usage, but it's not even located in the right place to signify the gap between "Truly" and "had".
You resolve this discrepancy by just pulling out the assumption from thin air that he didn't use the symbol correctly, which is no different from Walt assuming that he meant "and" and didn't use the symbol correctly. It also resembles a stylized plus sign which people use to indicate "and". Something like this:

(with the crossbar either faded or missing)
But the bottom line is that neither you or Walt can conclusively determine what Hosty meant by just looking at the note. In the absence of the ability to ask Hosty what he meant or hope that he later expounded upon them (it doesn't appear that he did), you have to consider what the other interrogation reports said as well as Warren Caster's and Roy Truly's testimonies. Walt will just say that they were all "damn liars" (including Hosty) and that his take on the handwritten note is the only legitimate interpretation. But you're not much better.
The testimonies of Caster and Truly support my claim, not Walt's.
The "crossbar" is not "faded or missing" because it's not an ampersand logo and it's not a plus sign; it is an inverted caret which never had a "crossbar".
You don't have to accept it, makes no difference. It's obvious (to anyone with the ability to interpret) what Hosty did.