Isn't "plan/intend to go" more appropriate for つもり？
It means the same thing in this case. Tsumori is used when you know for sure you're going.
It would be nice for new learners though if Duolingo made an effort to show the difference between Verb and Verb+つもり
Not sure that's right. You "intend" to go isn't the same as definately going. I had heard "yotei suru" (to plan / to schedule / to arrange) is stronger.
I got it wrong but actually tsumori in unneccessary here
It's not exactly necessary, but it's more natural to add tsumori.
Yes, I believe it makes sense because the event is in the future, so it's technically just a "plan". And of course there's no future tense.
Why is 行き written as 行く?
行く is the plain form and should be used with つもり and other such constructs.
This is madness.
Why is there no (not accepted) に particle after 今年？Is there a rule that says time expressions need no particle. Can you drop the 今年 in anywhere in the sentence?
Time expressions usually go first. Specific time expressions (like Tuesday) need the に particle, but I believe that に is optional for relative time expressions like today, yesterday, this year, last year, and so on.