As mentioned before, lot of questions (that you should be able to answer yourself if you read properly the upgrade guide and the referenced notes)
I will try to clarify as it seem that the answers are not complete. I will not answer in your order so it can be understand properly
2) Do we need to upgrade our SAP Kernel to version 7.22_EXT first ? Can it support non EXT?
What is the recommended kernel version we should go for?
you do not need to upgrade to 7.22_EXT, but you need to go to an EXT patch (for example 7.21_EXT) as your current kernel 7.21 is not supported with oracle 12c. You can upgrade before or after the upgrade (which is mentioned on the upgrade guide that you should have read)
Kernel 7.21_EXT needs oracle client 11 or 12, this anwsers your first question.
1) Do we need to upgrade our oracle instant client at least to 11.2.0.4 latest version first?
if you do not upgrade the SAP kernel first, then you do not need to upgrade the client first.
If you upgrade the kernel after oracle then use the oracle client 12c, not the 11g.
I do not agree with Willi Eimlerwhich mentioned:
You don't need to upgrade your instantclient, but the 11 instantclient support ends in a short time. I changed to the 12 instantclient.
you do need to upgrade the instantclient. 10g is not supported. kernel. 7.21_EXT needs client 11g or 12c
3) For BR*Tools 7.40, we only apply after we upgrade the kernel to eg. 7.22_EXT?
as mentioned you need 7.40 for Oracle 12c. you can upgrade them whenever you want as they can be used with any SAP kernel but keep in mind the oracle client they need
4) Do we need to setup SSF first before the upgrade?
you can set up it before or after, it is up to you. Check the requirements (11g, kernel 7.20)
5) We don't use ACO feature in our current oracle, I guess we can skip (SAP note 2157904), right?
yes,but you should have the support packages mentioned there.
6) Do we need use create the new user oracle to perform the out-of-place upgrade or use <sid>adm?
again, clearly the upgrade guide is not read as there is stated clearly that you still can use the "SAP classic" user concept
If you do all the things that can be done before the upgrade, the shorter will be your downtime.
and, typical recomendation, read the "fantastic" upgrade guide