Intuition for Simplicity's CheckSigHashAll

So, I downloaded Simplicity and started a REPL using cabal new-repl Simplicity. Then I enabled type applications using :set -XTypeApplications.

Consider these invocations:

> (pkwCheckSigHashAll @CommitmentRoot @() lib (Schnorr.PubKey True (read "0")) (Schnorr.Sig (read "0") (read "1")))CommitmentRoot {commitmentRoot = Hash256 {hash256 = "\CAN\r\201\231\SO0:1\183\130a\DC3m4u!\193\247e\EOT\194nO\US\208\150\&2\182\ACK\203\151>"}}> (pkwCheckSigHashAll @CommitmentRoot @() lib (Schnorr.PubKey True (read "1")) (Schnorr.Sig (read "0") (read "1")))CommitmentRoot {commitmentRoot = Hash256 {hash256 = "\215\222oj\251&\134\SOZ\202@N\161(\185j2\DELt\156\147\136Nz\183\179\EOTH\166\FS\141F"}}> (pkwCheckSigHashAll @CommitmentRoot @() lib (Schnorr.PubKey True (read "1")) (Schnorr.Sig (read "0") (read "0")))CommitmentRoot {commitmentRoot = Hash256 {hash256 = "\215\222oj\251&\134\SOZ\202@N\161(\185j2\DELt\156\147\136Nz\183\179\EOTH\166\FS\141F"}}

If I understand correctly, the commitment root goes into an transaction output. Since the signature shouldn't play a role, the output from Haskell makes sense since I change the sig and it doesn't affect the commitment root. The example above seems consistent with this intuition. Are both my intuition and the example code correct?

Now, I can call the same method with WitnessRoot:

> (pkwCheckSigHashAll @WitnessRoot @() lib (Schnorr.PubKey True (read "0")) (Schnorr.Sig (read "0") (read "0")))WitnessRoot {witnessRoot = Hash256 {hash256 = "\185\FS\176\179\b{Xc2\216\n\240\186\205v\208\164NW\\\DLE\193:\ETB\bMO\211\152*I%"}}> (pkwCheckSigHashAll @WitnessRoot @() lib (Schnorr.PubKey True (read "1")) (Schnorr.Sig (read "0") (read "0")))WitnessRoot {witnessRoot = Hash256 {hash256 = "\240h\NUL\188z4\ACK\200\ETX\151\DC1&Y\253\t\152\176P\146\186\137\NAKm!\STX\DC3\182\148\193\246\172O"}}> (pkwCheckSigHashAll @WitnessRoot @() lib (Schnorr.PubKey True (read "0")) (Schnorr.Sig (read "0") (read "1")))WitnessRoot {witnessRoot = Hash256 {hash256 = "\205\DEL\132Q\245\166!\196\178\248\136\194aO\243+\145T\200E\129I#\253F\134\173i\243K\154J"}}

Again, if I understand correctly, the witness root goes into the spending transaction input. It seems like the witness root is affected if I change either signature or public key, which corresponds to my intuition. Simplicity uses BIP-Schnorr which doesn't allow for pubkey recovery. Again, is my intuition consistent with the above code?

I assume these hash values contain mock transaction metadata like nLocktime, embedded in the lib, correct?

Now, let's say I wanted to do a 2-of-2 MAST-based multisig with Simplicity, how would that look like? There must be a way to compose two different calls to CheckSigHashAll. And the spending transaction would surely need to provide a MAST path through the script, how do I pass that to Simplicity? With oooh?