Preserving ownership of authorship in the period after leaving a position and before journal submission Background: 
I am currently working on a paper as a first author with 3 other members.
My tenure with the research institute ends on the end of this month and I have no intention to continue any working relationship due to the very complex political environment and "back-scratching" among upper management in this research institute I am in, plus my observations of other ethical issues.

However, I intend to submit this paper to a journal after the end of my tenure. This journal has an associated conference that has deadline submission on the 15th July 2018. For this particular conference, an acceptances of a paper for this conference would enable a direct proceeding to its associated journal. 
I have a valid concern aimed at the senior research fellow (No PhD), whom I report to and who is also the last author of this paper, due to strong corroborated anecdotes (credit stealing included) from multiple employees. As far as facts is concerned, he comes from industry after 20 years, without a PhD, and sorely lacks technical skills in my observation. 
This further cements my concern and I am aware that there exists a possibility that my concerns may be misguided.

Measures taken:

1) I have created an official email confirming the roles, responsibilities and order of authorship of each team member involved in this project. The email has been acknowledged by each individual team member including aforementioned senior research fellow/ programme director (without a PhD).
His role is to review to review the draft before my submission (possible and probably potential red flag?).
He has not acknowledged nor mentioned anything in his reply about getting back to me with changes to the draft. 

2) I intend to submit my paper to an open journal arXiv before submitting any draft to this senior research fellow.

3) 2) is followed by a submission of a draft to senior research fellow with a co-correspondence to the integrity office, worded in a diplomatic manner.

Are these three steps sufficient? What more should I be aware of?

Edit: The email also serves as a centralised communication where any work done by must submitted to that email.
In the world of patent, researchers establish priority in the following way. They keep a bound notebook (no loose-leaf) so that pages can't be added between other pages. Whenever they have a good idea for which they want to establish priority they make a dated complete note of the idea. They then get a colleague to date and sign their note. At some companies most employees always have this patent book with them so they can record ideas as they occur. 

Then, if another person somewhere tries to patent the idea, they can present the patent book as evidence of prior art. The other person in this case may be completely unknown to you, but for patent, priority is pretty much the whole game. 

You can do something like this yourself, if you are unsure. It needn't be confrontational to your co workers. Make a signed and dated (paper) copy of each version of the paper that you work on and have a non-involved but trusted colleague sign and date it. You can even leave them with a signed and dated copy so that you each have one. 

The existence of these copies protects you a bit from being excluded. But in the absence of a problem, they are just private notes and so non-confrontational. 

For your future, you are likely better off with cordial relations with your co-authors. It may even turn out that they are proud of their association with you.