I'm sorry but I don't understand the question to give you a full detailed answer. The only inference I can give you from your question is that your city defense spiders will get 20% on garrasion duty. Then if your city is in the desert it will be 40%.
yes. it is testable. sort of. For any race who just got a terain bonus, you can test just before you unlock it against 0 Def wall. and again just after you gain the perk. you will see a 17 to 23 % increase in total damage done. As for the garrison bonus. im not sure how to test if it actualy works, and or if it stacks with terain bonus.
the only way i can think to test garrison perk is to make a custom wall, with 1 spider on it. and you attack it with 1 jaques. and log the time it takes to kill the spider. then test again with a new spider with both perks, and note the time it takes to kill the spider again. if the spider survives longer..the perks work. or at leas one does. you may be able to get an estimate on % increase from the the time to kill. if its in the 20 range. one worked. if in the 40 range, both workerd and stacked