01) I am an international student, studying my Masters degree in
Tallinn, Estonia. self funding my studies.I am in my final semester.
what documents should i provide which satisfy the Uk visa authorities
that i won't slip in uk and will come back after the proposed time
spent in the Uk?
They got you on funds parking, it's a very common refusal ground. Please see Should I submit bank statements when applying for a UK Visa? What do they say about me?
There is also some interesting background information about this in the site's archives at What is 'funds parking' in the context of UK visa applications?
02) i have completely forgotten to mention in the application that i
have applied before in 2007 for studies in the UK, and received
refusal at that time as well. do i need some documents to clarify this
or should i just mention that in the application the cause of the
refusal where it has been asked in the application?
People who forget to properly record something in their application have a very difficult time going forward. While clerical errors can occur like the omission of a single check mark, yours left an entire section of the form blank and trying to explain it as simply forgetting something isn't going to work very well.
Why? If you previously applied in 2007, it means you were at least 18 at that time and this means the refusal is in your adult memory and if your brother is a UK permanent resident you would have extra impetus to be aware of your own history vis-a-vis the UK. Regardless if you truly forgot or not, from their point of view they would say that it's more likely you forgot about giving your biometrics and/or gambled that a new passport along with an EU class "D" visa would carry your application.
03 my reason to obtain visit visa is to meet my brother, who is a
permanent resident in UK. though i did present his bank statements and
his passport copy, but could not provide an invitation letter from
him. what do you suggest in this scenario?
The ECO does not believe you and in the context of the application as a whole there is little benefit by adding an invitation letter from your brother; they would most likely dismiss it as contrived given the tone of the refusal notice.
04)if i need to reapply for the same purpose of travel, should i use
the same account which i have made for the previous application for UK
visa online application?
Yes, you can use the same account. To do this you can click on 'delete application' for your current application, and then click the options to start a new application. Alternatively you can edit/update your application and submit it again.
Important: Because they got you on Paragraph 3.6 of Appendix V, they have the option, but not the obligation, to issue a ban if you apply again. Explaining away your false representations is not a matter of a blithely telling them you forgot. And in the best case they will use the discrepancy method to refuse again. The "best practices" advice when there are serious problems is to show your refusal to a UK solicitor who can make representations on your behalf with a fresh application. It will be expensive in your case because they got you on funds parking and false representations.
Note: See also What can I do to lift a 10 year ban for making a mistake in my UK visa application?
Note: See also Deception
Note: "Discrepancy Method" in this context means to aggressively hyperbolize any sort of deficiency when there is an opportunity to do so. This method can be used for high-risk applicants with perceived or real credibility problems.