How about expanding the discovery procedure with a NAPTR request sent to the provider's domain (as found in From and/or P-A-I) in case (1) fails

Yes, looks as it will be a good extension.
Domain from From header may be used only if domain not available in P-A-I header.

Here examples of From and P-A-I headers that we can receive
Example 1 - domain matched in From and P-A-I headers

From: <sip:+18556667788@206.147.246.118:5060>;tag=gK0e0d09a4
P-Asserted-Identity: <sip:+18556667788@206.147.246.118:5060>

Example 2 - private IP address in From header and no domain in P-A-I header. P-A-I contains tel and sip uri.

From: <sip:+16692481432@198.226.45.84>;tag=gK04090b5e
P-Asserted-Identity: <tel:+16692481432>, <sip:+16692481432@vzimstest.com>

Example 3 - IPv6 address in From header and no domain in P-A-I header

From: "9255111003"<sip:9255111003@[2605:97c0:2058:3120:1::1]:50001>;tag=75fdbcf61a;epid=SC597fbd
P-Asserted-Identity: <tel:9255111003>

Example 4 - no domain in From and P-A-I headers

From: <tel:+13559598013>;tag=SDtl3t701-5f7f754a-5f89c26229204b60-Noo-lucentNGFS-000264
P-Asserted-Identity: <tel:+11005534340>

Think not required to make double reverse and NAPTR DNS lookups if the domain from P-A-I or From headers equals to caller network IP address.

Could you look at this example.

P-Asserted-Identity: <tel:+16692481432>, <sip:+16692481432@vzimstest.com>

IP address does exist here, put present caller domain. So we can skip reverse lookup and execute NAPTR.

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