Can this sentence mean both "walk in the village" and "walk to the village" ?
A location used with kutembea is generally the location, not the destination, so it's basically "Walk in the village."
To say "to", you can add the word kwenda:
Tembea kwenda kijijni. = Walk to the village.
Why are so many expressions here seemingly wrong ? A Kenyan friend of mine told me that sometimes one should use ongea instead of sema, kiasi instead of kidogo, that one shouldn't omit the ku- in kunywa, and many other things. Are the expressions that are being used here some sort of dialectical expressions different from standard Swahili? And which place speaks the so-called standard Swahili?
This course generally teaches Tanzanian Swahili, which, as I understand it, is considered more standard.