SSM Keybinds (unminified)

keybinds.js

let dropWrapBlockSelection = 0
let currentlySelectingSubblock = false
let currentlyUsingBlock

function escapeFromDropwrap()
{
    // stop everything and escape from the dropwrap
    let dropWrapChildren = currentlyUsingBlock.parentElement.children
    dropWrapChildren[dropWrapBlockSelection].style.backgroundColor = ""

    dropWrapBlockSelection = 0
    currentlySelectingSubblock = false
        
    currentlyUsingBlock.parentElement.dispatchEvent(nme("mouseout"))

    document.querySelector(".using").style.backgroundColor = ""
}

function nme(eventName)
{
    // creates a new mouse event and returns it
    return new MouseEvent(eventName, {
          "view": window,
          "bubbles": true,
          "cancelable": true
    })
}

function getDWC()
{
    // returns the dropwrap's children
    return currentlyUsingBlock.parentElement.querySelector(".drop").children
}

document.addEventListener("keydown", event => {
    if (event.code == "KeyE") document.querySelector(".erase").click()
    else if (event.code.startsWith("Digit"))
    {
        // click on specified block in palette
        let number = Number(event.code.replace("Digit", ""))
        let bElems = document.querySelector(".building-elements")
        let element = bElems.children[number]
        try {
            element.click()
            element.querySelector("a").click()
        } catch(_) {}
    }
    else if (event.code == "ArrowDown" && !currentlySelectingSubblock)
    {
        // open sub-blocks in the dropwrap by simulating a mouseOver
        currentlyUsingBlock = document.querySelector(".using")
        if (currentlyUsingBlock.parentElement.classList.contains("drop-wrap"))
        {            
            currentlyUsingBlock.parentElement.dispatchEvent(nme("mouseover"));

            currentlySelectingSubblock = true

            getDWC()[dropWrapBlockSelection].style.backgroundColor = "red"
        }
    }
    else if (currentlySelectingSubblock && (event.code == "ArrowDown" || event.code == "ArrowUp"))
    {
        let prevSelection = dropWrapBlockSelection
        
        // scroll up or down in the dropwrap
        if (event.code == "ArrowDown") dropWrapBlockSelection++
        else dropWrapBlockSelection--

        // update

        let dropWrapChildren = getDWC()

        if (!dropWrapChildren[dropWrapBlockSelection])
        {
            // oopsies, scrolled past the end, or before the start of the list
            if (event.code == "ArrowDown") dropWrapBlockSelection--
            else dropWrapBlockSelection++
            return
        }
        
        let mouseover = nme("mouseover")
        let mouseout = nme("mouseout")
        
        dropWrapChildren[prevSelection].dispatchEvent(mouseout)
        dropWrapChildren[dropWrapBlockSelection].dispatchEvent(mouseover)

        dropWrapChildren[prevSelection].style.backgroundColor = ""
        dropWrapChildren[dropWrapBlockSelection].style.backgroundColor = "red"

    }
    else if (currentlySelectingSubblock && event.code == "Enter")
    {
        // yay, finally selected a block in the dropwrap
        
        getDWC()[dropWrapBlockSelection].click()
        
        escapeFromDropwrap()
    }
    else if (event.code == "Escape")
    {   
        escapeFromDropwrap()
    }
})

Minified code:
(()=>{let e=0,r=!1,o;function t(){o.parentElement.children[e].style.backgroundColor="",e=0,r=!1,o.parentElement.dispatchEvent(c("mouseout")),document.querySelector(".using").style.backgroundColor=""}function c(e){return new MouseEvent(e,{view:window,bubbles:!0,cancelable:!0})}function n(){return o.parentElement.querySelector(".drop").children}document.addEventListener("keydown",l=>{if("KeyE"==l.code)document.querySelector(".erase").click();else if(l.code.startsWith("Digit")){let i=Number(l.code.replace("Digit","")),d=document.querySelector(".building-elements").children[i];try{d.click(),d.querySelector("a").click()}catch(s){}}else if("ArrowDown"!=l.code||r){if(r&&("ArrowDown"==l.code||"ArrowUp"==l.code)){let u=e;"ArrowDown"==l.code?e++:e--;let a=n();if(!a[e]){"ArrowDown"==l.code?e--:e++;return}let p=c("mouseover"),y=c("mouseout");a[u].dispatchEvent(y),a[e].dispatchEvent(p),a[u].style.backgroundColor="",a[e].style.backgroundColor="red"}else r&&"Enter"==l.code?(n()[e].click(),t()):"Escape"==l.code&&t()}else(o=document.querySelector(".using")).parentElement.classList.contains("drop-wrap")&&(o.parentElement.dispatchEvent(c("mouseover")),r=!0,n()[e].style.backgroundColor="red")})})();