All subjects are Equally important dear and required equal contribution. But generally Students concentrates more on Numrical Subjects and give less time to Theyoritical Subjects. So, we can answer your questions as follows,
(1) Numrical Subjects Like both Accounts and Costing & FM requires lots of practices (and yes past papers questions must be solved). So, give more time. In my opinion COSTING & FM requires more time then ACCOUNTS.
(2) Taxation is partly theory and partly practical so, work more on understanding rules and there applications and yes don't forget to read latest amendments applicable for exams.
(3) For theory give time to understand the concepts and here INFO TECH & SM requires more hard work then other theorical subjects so plan and work accordingly.
Don't think about failure do smart work for clearing both groups in once and yes passing of either group does not require to repeat it means, an student only requires to give the group in which S/he fails.
Your last question answer is, Yes you can give your Second Group during Articleship by taking sufficient leave all depends on your planning and capacity, but in my opinion try to clear both group in once and if possible join any coaching not for full course but for tests etc.
Hope above information helps you. For further doubt please comment.
With Best Wishes,
Abhi