I tried removing the LF characters as suggested by you, but that didnt work. So, I used the certificate and key files AS given by AWS itself. Here is the log during successful execution. It has answers for your other questions.
[2024-08-28_13:30:52:422]AT
[2024-08-28_13:30:52:422]OK
[2024-08-28_13:30:57:434]AT+CGREG?
[2024-08-28_13:30:57:434]+CGREG: 0,1
[2024-08-28_13:30:57:434]OK
[2024-08-28_13:31:00:952]ATI
[2024-08-28_13:31:00:952]Quectel
[2024-08-28_13:31:00:952]EC200U
[2024-08-28_13:31:00:952]Revision: EC200UCNAAR03A08M08
[2024-08-28_13:31:00:952]OK
[2024-08-28_13:31:04:977]AT+CGREG?
[2024-08-28_13:31:04:977]+CGREG: 0,1
[2024-08-28_13:31:04:977]OK
[2024-08-28_13:31:09:755]AT+QICSGP=1,1,“airtelgprs.com”,“”,“”,1
[2024-08-28_13:31:09:755]OK
[2024-08-28_13:31:12:710]AT+QIACT=1
[2024-08-28_13:31:12:710]OK
[2024-08-28_13:31:15:866]AT+QIACT?
[2024-08-28_13:31:15:866]+QIACT: 1,1,1,“100.87.53.157”
[2024-08-28_13:31:15:866]OK
[2024-08-28_13:31:20:343]AT+QMTCFG=“SSL”,0,1,2
[2024-08-28_13:31:20:343]OK
[2024-08-28_13:31:23:945]AT+QMTCFG=“recv/mode”,0,0,1
[2024-08-28_13:31:23:945]OK
[2024-08-28_13:31:29:430]AT+QFDEL=“cacert.pem”
[2024-08-28_13:31:29:446]OK
[2024-08-28_13:31:32:497]AT+QFDEL=“client.pem”
[2024-08-28_13:31:32:497]+CME ERROR: 418
[2024-08-28_13:31:35:559]AT+QFDEL=“user_key.pem”
[2024-08-28_13:31:35:559]+CME ERROR: 418
[2024-08-28_13:31:42:773]AT+QFUPL=“cacert.pem”,1187,100
[2024-08-28_13:31:42:773]CONNECT
[2024-08-28_13:31:50:193]+QFUPL: 1187,2d19
[2024-08-28_13:31:50:193]OK
[2024-08-28_13:32:13:372]AT+QFUPL=“client.pem”,1220,100
[2024-08-28_13:32:13:372]CONNECT
[2024-08-28_13:32:16:282]+QFUPL: 1220,6737
[2024-08-28_13:32:16:282]OK
[2024-08-28_13:32:36:215]AT+QFUPL=“user_key.pem”,1675,100
[2024-08-28_13:32:36:215]CONNECT
[2024-08-28_13:32:37:597]+QFUPL: 1675,0137
[2024-08-28_13:32:37:597]OK
[2024-08-28_13:32:47:808]AT+QSSLCFG=“cacert”,2,“UFS:cacert.pem”
[2024-08-28_13:32:47:808]OK
[2024-08-28_13:32:51:181]AT+QSSLCFG=“clientcert”,2,“UFS:client.pem”
[2024-08-28_13:32:51:181]OK
[2024-08-28_13:32:54:652]AT+QSSLCFG=“clientkey”,2,“UFS:user_key.pem”
[2024-08-28_13:32:54:652]OK
[2024-08-28_13:32:58:298]AT+QSSLCFG=“seclevel”,2,2
[2024-08-28_13:32:58:298]OK
[2024-08-28_13:33:01:188]AT+QSSLCFG=“sslversion”,2,4
[2024-08-28_13:33:01:188]OK
[2024-08-28_13:33:04:641]AT+QSSLCFG=“ciphersuite”,2,0xFFFF
[2024-08-28_13:33:04:641]OK
[2024-08-28_13:33:07:270]AT+QSSLCFG=“ignorelocaltime”,2,1
[2024-08-28_13:33:07:270]OK
[2024-08-28_13:33:10:618]AT+QMTOPEN=0,“xxxx”,8883
[2024-08-28_13:33:10:618]OK
[2024-08-28_13:33:11:985]
[2024-08-28_13:33:11:985]+QMTOPEN: 0,0
[2024-08-28_13:33:24:585]AT+QMTCONN=0,“mydevice”
[2024-08-28_13:33:24:585]OK
[2024-08-28_13:33:24:899]
[2024-08-28_13:33:24:899]+QMTCONN: 0,0,0
[2024-08-28_13:33:37:585]AT+QMTSUB=0,1,“test”,1
[2024-08-28_13:33:37:585]OK
[2024-08-28_13:33:38:103]
[2024-08-28_13:33:38:103]+QMTSUB: 0,1,0,1
[2024-08-28_13:33:43:288]AT+QMTPUB=0,1,1,0,“test”
[2024-08-28_13:33:43:288]> {“message”:“Hello from device”}
[2024-08-28_13:34:11:100]
[2024-08-28_13:34:11:100]OK
[2024-08-28_13:34:11:398]
[2024-08-28_13:34:11:398]+QMTPUB: 0,1,0
[2024-08-28_13:34:11:398]+QMTRECV: 0,1,“test”,33,“{“message”:“Hello from device”}
[2024-08-28_13:34:11:398]”
[2024-08-28_13:34:30:345]
[2024-08-28_13:34:30:345]+QMTRECV: 0,0,“test”,45,“{
[2024-08-28_13:34:30:345] “message”: “Hello from AWS IoT console”
[2024-08-28_13:34:30:345]}”