So zapping will now require me to tab out to a different app to approve a spend?
Only if that's what your wallet setup/settings require. Mine doesn't. That's up to you.
So zapping will now incur lightning fees?
That's right. If fees are too high for you, you can always use cowboy credits instead.
If the recipient is offline (say a mobile wallet), the zap will fail?
If all the recipient's attached wallets fail (you can attach more than one), we will fallback to giving the receiver fee credits unless they indicate they'd prefer to receive nothing if no wallet is available.
Why are some wallets send only (like Blink)?
Some wallets only support one or the other. For some wallets like Blink, we've only implemented one or the other. We plan on implementing Blink receives soon (@rblb mentioned wanting to work on it this morning).
I like Blink as an intermediary because it obfuscates my identity (I don't reveal my main LND node, which I may want to associate with a different identity).
For non-custodial p2p zaps on SN, we implement an lnproxy like forwarding mechanism. So the only one that would learn your lnd node's identity is SN. That still might not suit your threat model, but it's worth mentioning.