Earlier process (moved out of main issue description)

I think I should be able to pass a solana Instruction from the rpc directly to the exported serum function decodeInstruction()

I tried with raw TransactionInstruction from getConfirmedTransaction...
and with:
PartiallyDecodedInstruction from getParsedConfirmedTransactions

both give an error with this stack:

 Layout.js:570 Uncaught (in promise) TypeError: b.readUIntLE is not a function
    at UInt.decode (Layout.js:570)
    at OffsetLayout.decode (Layout.js:529)
    at UnionLayoutDiscriminator.decode (Layout.js:1414)
    at Union.decode (Layout.js:1682)
    at VersionedLayout.decode (layout.js:97)
    at decodeInstruction (instructions.js:108)

it seems that instructions.decode is passing along the whole (untyped) message and later decode is expecting it to have a
readUIntLE function.

further details of the issue:

@armaniferrante said
That decodeInstruction takes in raw bytes, i.e. the "instruction data"

So rather than passing the whole instruction one needs to pass the instruction.data in as a Uint8Array of raw bytes.

In this codesandbox i'm using basex-encoder.decodeToBuffer() to convert the PartiallyDecodedInstruction.data (which is base58 encoded) and then passing that to the serum decodeInstruction() fx, which returns this:

newOrderV3: Object
  side: "buy"
  limitPrice: BN
  maxBaseQuantity: BN
  maxQuoteQuantity: BN
  selfTradeBehavior: "decrementTake"
  orderType: "limit"
  clientId: BN
  limit: 65535