Your query escapes me sorts of so I divided your post and will try to address single
1) Look up the wind, current data for the passage, month. Then look up your boat's polar and combine the data. Add 50% as a safety
margin. There are computer programs that will calculate the THEORETICAL passage times for you.
2) what implications do you have in mind?
3) no way a boat with hull speed
of 5 knots will average her hull
speed on a passage, and especially so downwind. Downwind tends to be the slowest, the least comfortable, point of sail too.
4) ??? Why should this be? You want to carry water
for the planned passage AND a safe margin. The only excuse not to do so I can think of is when you are participating in some sort of extreme sports event. If so, please make sure your insurance
costs from mid-ocean. Sailing with exact, no margin, amounts of food
is a asking trouble.
5) a) you do not have to sail downwind if the destination
is downwind, you may elect broad reaching which may result in better VMG, b) take your boat on a shorter downwind passage, fully loaded and measure her speed over 24 hours, allow for the current. This will be a good starting point to estimate the possible passage times. c) look up passage times on the same route by boats with similar characteristics.
Now, you must also allow for the fact that stat data (meteo and boat's) may and will considerably vary each side of the average. Allow for that.
Make sure your wind-pilot and auto-pilot are there for you. You can only keep up good speed if the boat is sailed. Lay a-hull to get some sleep and the averages drop way down.
From my limited experience and minding the limitations of our smallish boat I can tell you that a boat with 6.5 knots of hull
speed can average 3.8-4.8 knots on an extended downwind, tradewind passage - a small boat (23' LWL), loaded with supplies and gear
, sailed well, but conservatively. Bigger boats may reach their upper speeds easier as they are less loaded and stand up to wind and sea better.
PS The Cocos - Rodriguez is a bad example as this stretch of the ocean is usually way more windy than other offshore
downwind passages. Many boats sail in the uppers of their potential for most of the Indian passage, ban the take off and the landfall.