Hybrid Migration from Exchange 2016 to office365

Hi MS,

I am in the planing phase of doing 10 users Pilot Hybrid migration to office 365 in E3 plan using our Volume licence.

We have two 2016 exchange servers running in DAG

We also have email filtering done via Mimecast and for purpose of this exercise i want to leave email filtering to mimecast and not use EOP.

I want to get a clear idea of exact steps I need to take to configure this.

However, before I do anything i want to make sure it does't affect our SMTP or any other users from reading tons of articles and some previous experience with office 365 this are the steps i think needs to be performed.

1. use exchange server deployment assistance

 - check prereq - all good

My questions  Questions are 

On step 14

If i add this TXT record this will not affect any mail flow this is purely for domain verification? 

Question 2

on the step 15 -29 here 

https://technet.microsoft.com/en-us/exdeploy2013/Checklist?state=3229-W-EwAIAAAAQAAAAAEAAAAAACAAAABEaAOFAAA~

will that affect any existing mail flow or existing connectors as we have few there

Question 3 

overall understanding is mailflow will still be going through exsting exchange server and other 240 users will not be affected and not other server or services such as smtp, scanning service will be affected due to this. 

We never need to change MX records and we don't have to make any changes to mimecast?

Please clarify these ASAP, would be really appreciated.