I understand that a channel is a 2 of 2 multisig.
Right, a channel according to the current Lightning protocol must be a 2-of-2 multisig. As far as the blockchain is concerned any funds moved from this address require two signatures from the two specified public keys.
However, as you rightly say Schnorr multisig (e.g. MuSig2) and Schnorr threshold key aggregation schemes (e.g. FROST) allow you to simulate multisignature and threshold signatures arrangements with only one signature and one (aggregated) public key going onchain. So you can take one (or both) of those 2 keys in the 2-of-2 and do as you suggest.
In addition the Lightning protocol could change in future to take advantage of MuSig2 and turn that 2-of-2 into a single pubkey spend. However, that most likely won’t happen anytime soon, requires a Lightning protocol change, channels to upgrade etc. The nested schemes you suggest can happen today and as far as the blockchain and the Lightning gossip is concerned it is a valid 2-of-2 with no knowledge of the simulated arrangement behind it.
Separately, I presume if you use Schnorr signatures, there is no way to know how many are needed for the threshold. Is this true? So it would be:
Right, unless you are part of the multisig or threshold arrangement all you see is the signatures on the blockchain. So you don’t know whether a threshold was used at all and if it was what exact threshold it used.
Are these live on LN?
As Rene says we expect this isn’t happening today as we haven’t seen open source implementations of it yet but it is possible it is.