Learn Farsi: گرده یک عامل مهم برای تکثیر محسوب می‌شود. - Pollen is considered an important factor for reproduction.

With the verb محسوب شدن (to be considered or counted), you don't strictly need a preposition like به عنوان. You can directly equate the subject (گرده) with its complement (یک عامل مهم), just like in English where we often say 'is considered an important factor' rather than 'is considered as'.

How can I attach a sentential relative clause to this sentence to say '..., which is essential for plants' using this lesson's grammar?

You can add ، که این امر برای گیاهان ضروری است to the end of the sentence. The phrase که این امر (literally 'that this matter' or 'that this act') acts exactly like the non-restrictive 'which' in English, allowing you to comment on the entire preceding fact about pollen without altering the main sentence.

Is تکثیر the only word for 'reproduction'?

تکثیر specifically means multiplication, propagation, or proliferation, making it perfect for plants or cells. For human or animal reproduction, you will more often hear تولید مثل (literally 'producing likeness').

What is the active equivalent of محسوب می‌شود if I wanted to say 'Scientists consider pollen an important factor'?

You would use the active verb محسوب کردن (to consider). The sentence would be: دانشمندان گرده را یک عامل مهم محسوب می‌کنند. Notice how گرده now takes the direct object marker را because it is the specific thing being acted upon in an active sentence.