Meaning of the hadith “War is deception”

Q: Abu Hurairah and Jabir (may Allah be pleased with them) reported that the Prophet (ﷺ) said, “War is deception.” [Al-Bukhari and Muslim] 

Is it permissible to deliberately spread a false rumour accusing the wife of a prominent political enemy of the Muslims of adultery, with the intention of discrediting him, causing turmoil and distress in his personal life, and thereby hoping to gain a strategic advantage for the Muslims?

Actions to repeat after re-entering the fold of Islam

Q: If someone does kufr or shirk (but later makes tawbah and comes back), do they have to re-do all of the following? 

- previous valid zakat 

- previous valid fitrana+qurbani 

- previous valid prayers + fasts (what if they were in the middle of making up many years worth of qadas?) or can they ignore it and focus on the present and future acts they must do?

Calculating the value of gold for previous years' unpaid zakaat

Q: I wanted to ask some things related to the payment of zakaat. They are as follows: 

I was unaware that I had enough gold previously and now I have to pay zakaat for the last 2.5 years. So will the zakaat of each year be calculated according to the gold value in that year, or according to the current year's gold value? 

If I won't pay it all at once, and do it in installments will I be sinful in delaying it further?