Finland has one of the highest success rates for multiple-entry visas (about 80%).
If you envision multiple visits and you have a good premise to support multiple visits, then you should apply for a validity of 1 year and use the Schengen 90/180 rule to govern your actual visits. This advice would be true even for your first application.
There is no risk in asking for a year. If they do not think you qualify for a year, they will downsize the validity to something they are comfortable with. If they grant you a year, then you have saved.
See the META thread: Unanswered or dubious answers on Schengen visa questions for some background on this question. And remember that your premise is key-critical.
For your other questions...
And what should I mention in my cover letter as my sole intention is
Get real. The overwhelming consensus is that you tell them the truth. Explain your intentions with natural language and use a translator if needs be to assure that it is clear and crisp.
Also, for multiple entry do I need to have insurance of complete
duration of one year for which I am applying or is it enough if I have
insurance for the ten days of my first visit?
Get coverage for your initial visit. When the border officials see a multiple visit visa they may challenge you for insurance so be prepared for it.