Thank you very much for your kind help
Regarding A), this means that given a transaction such as below:
{"txid":"d77a241272a04cdcf7513cca04e83dccbf56de5fe602ab2e13f5a0079aa3d711","hash":"23f9da398c98c9be73c0799d54c31c748e5a32e69539e038db13c1f875448c2d","version":2,"size":388,"vsize":226,"weight":904,"locktime":0,"vin":[{"txid":"aac4568857f39a5dcfbcd7be65197a118e1ab1c1dd04566837aceebbf9d3eb4e","vout":0,"scriptSig":{"asm":"0014f5db3a1acc7f18405ab1db8ab54344336d876882","hex":"160014f5db3a1acc7f18405ab1db8ab54344336d876882"},"txinwitness":["304402207949fa75657d5df84c7ed3fadffa10747ce5c7958e1aee3df42ef3fc985331010220552 3cb3b2dd46a6e1c42993b562ba3f530948fec8652d1bd6610c7e97f313a5801","02c787917771dbb98951124b2cc22fe7612f1e1610a79620c48473f8c7dd67f456"],"sequence":4294967295},{"txid":"cfe79aec8fc90f278baf74df75065899a1e933b74f7d6f89ae260a84002c0557","vout":21,"scriptSig":{"asm":"0014bdd7544ed9c4dfb56f8bd786cf81f6344da6e663","hex":"160014bdd7544ed9c4dfb56f8bd786cf81f6344da6e663"},"txinwitness":["3044022036cb3c6f6e426d57f2e41ff1f558f6bae6ebf8a50d38fdb7a15e3558de45f92b02205c3 4f86d2210f6902fb97aa1c419745d4fcf468529721113ec0522eeded0e3f901","0206ebc953a6090d2476879a2c31761e03f7ccedb824feeeec78305ae267bc40bc"],"sequence":4294967295}],"vout":[{"value":0.16414018,"n":0,"scriptPubKey":{"asm":"OP_DUP OP_HASH160 aa97701c85ef9d651e4e9bf01bf7330f2e03e7bc OP_EQUALVERIFY OP_CHECKSIG","hex":"76a914aa97701c85ef9d651e4e9bf01bf7330f2e03e7bc88ac","reqSigs":1,"type":"pubkeyhash","addresses":["1GZ1HhoA3ijNZzsSjDPBgtwenRZvXZbR48"]}}]}
I should be able to know the signatories public keys (the same way miners check the validity of the transaction if I am not mistaken) and I then should try determining to which Xpubs those public keys are linked (if a HD wallet is used)