Currently, there's no way to reduce the sync time other than to get a copy from someone else/another device of yours or get better internet. Though Lightning nodes can run on Neutrino which is a light client until the sync is complete.

At this moment, Lightning gossip seems to be not too big of an issue. When it does get too big though, the only solution is to reduce channel updates and/or public channels.

When a normal (no hosted channels or custodial services) channel gets closed, funds are sent according to the last state at the time of closure to the corresponding parties. So if you had 50000 sats in a channel and it closed, you get 50000 sats minus chain fees.

Non custodial as connected to your own node: yes! You can create LNURLw links from your own node using LNbits, create an LN address and/or LNURLp link using LnMe/BTCPay/LNbits/... and so on

No idea. Probably at least 2-3 years.

Blinded paths work by providing the sender blinded node IDs to encrypt onion data to + an encrypted blob for every hop that the recipient creates that contains the real route information.

HTLCs use a hash of a preimage as the lock and the preimage as the key and PTLCs use a public key (point) as the lock and a signature from that public key as the key, see https://bitcoinops.org/en/topics/ptlc/

well I can't list them all here :(