There is a transcript of a previous discussion related to descriptors that has similar ideas on encoding: Bitcoin Core Dev Tech 2024: Silent Payment Descriptors.
For (3), let me add that b_scan and b_spend are not enough to spend outputs by themselves, as they require the tweak material from the original transaction creating the output. Shouldn’t we also consider a format for “self contained” spending material for individual outputs? like tweak + b_spend , where tweak = label + b_scan + shared secret or even b_scan + b_m + shared secret.
For (2), it may also be useful to encode a range or limit for the number of applicable labels on the string itself.