Can ECO reject my case, and ban me for 10 years, this time by saying that I used deception and withhold relevant information?
I think the chances of another refusal are high, but your application will not attract a 10 year ban on deception. It's a judgement call as to whether an IELTS certificate constitutes 'compelling new evidence', but omitting this evidence from an earlier application would not be considered an aggravating factor. The law requires an aggravating factor (or something that would have resulted in a refusal had the ECO been aware of it). If you have the IELTS proficiency and are simply establishing it, then you are off the hook. On the other hand, if you claimed to have the IELTS proficiency and actually did not, or submitted a forged certificate, they would get you for deception and yes, that situation invites a 10 year ban under Paragraph 320.
As explained here, three successive applications in a very short period of time with virtually no change in circumstances and a marginal improvement in evidence raises the question of 'frivolous applications', and this invites a ban, but not a 10 year one and only if they decide that your applications are frivolous.
Is it wise to lodge the complaint now or shall I wait for the application outcome?
It makes no difference. I have used their complaints procedure literally dozens of times (not for myself, on instructions from clients), and it honestly makes no difference when it is raised. They use segregated streams for complaints received at Marsham Street or Lunar House, but you can also complain directly to the consulate and get the local staff involved. I concede that people are paranoid about it because they fear it will adversely affect their application, but it doesn't happen. Unprofessional behaviour is a sackable offense and few would even consider it.
Your point about how the ECO should not be minded if you have sufficient funds or not might be one for debate some time. But in the current era the ECO is supposed to take that information into account. It's part of 'what is'. And your point that they did not receive (or apparently did not receive) your IELTS certificate might be a good one if in fact it was never logged in to Proviso or if the VFS staff removed it before passing your application to the consulate (they do that sometimes).
If the VFS staff removed it, somebody will get spanked and it will make no difference to your application.
Even without lodging a complaint, you can always enter a remark in your next application along the lines of...
In my previous application(s), the ECO did not examine evidence that
was favourable to me and had he done so the decision would have been
You should read the UKVI Complaints Procedure information and send your complaint via email (do not use the form, it's not the right vehicle for your type of complaint). If you are planning to complain directly to the consulate, you can deliver it to the VFS at the same time as your application and save yourself a trip.
One more thing that you didn't ask, but is relevant here when you are sending stuff to a British consulate as evidence for a visa application. It is best practices to make a fully itemised list of your evidence...
- Document name
- Original or certified copy or ordinary photocopy
- Document date(s)
- Number of pages
- Why it should be examined
This is not a required step. It is 'best practices', i.e., what a lawyer would do if he were representing your application. Taking this step reduces the chances of missing evidence, but more importantly shows that you are a coherent and organized person. It might even carry the app. They love that stuff. They eat it up.
NOTE: The OP's previous question is here.