And how are they going to securely send the customer the seed and public key to the multisig wallet without it getting intercepted by an attacker? Someone getting possession of it will neutralize the security of using multisig in your plan.
Are you trying to say that sending public keys over HTTPS is not secure somehow? Also, intercepting public keys could only harm privacy, but still, private keys are needed to spend it. Also, multiplying and dividing keys is enough to prove the ownership of some private key and share the public key at the same time. More than that: both private keys are needed (or both signatures signing some transaction chosen by the attacker). If only one would be compromised, then altering transaction by some third party is impossible.
unless you are mining at a pool, but then there's some form of centralization here, they can almost be considered a "cloud" too
Exactly. It could be useful to receive some smaller amounts by forming some kind of CPU pool. For example: if mining 1 BTC needs a lot of power, then mining single satoshi is 10^8 times easier. Then, if there are many clients, that could be promising somehow. "Cloud" or "pool" mining operator can just receive shares and send payments in this "LN-like way". Sooner or later, millions of such miners could produce a block.