@Durin
I know this is a huge number of questions, but they're all important.
1. Did the Eldar bring any exterminatus weapons?
2. Is the total number of well fortified/populated/defensively led Ork worlds and Chaos Agriworlds(worlds that will provide the an especially large increase in biomass) in the path of the Tyranids that have a webway gate greater or lesser than the number of exterminatus weapons that we could use against them?
3. Would channeling resources into fortifying those of our worlds that the Tyranids are going to first strike be useful?
4. What level could we improve those defences to in our approximate timeframe?
5. Actually how many of these planets being struck can we get a fleet to through the webway?
6. Are there more we could move ground forces to through the webway?
7. can the Eldar manually blast a planet sterile in less time than we can?
8. When you say it would take a year to be sure a world is sterilized by orbital bombardment, how large a fleet detachment would that take?
9. If we assasinate the Ork warboss attacking Amir Ka(to make their fleet break and run) could we support another one to take control before the Hive fleet arrives?
10. Do the Eldar know anything about the local necron dynasty? Like whether they're likely to intervene against the Tyranids, what forces they have, whether we should prioritize holding some exterminatus weapons in reserve to have a chance against them in a future war, whether such a war would be likely after they finish the War in the Void.
11. Do their seers think there is any external force or conspiracy attempting to arrange things to compromise attempts to stop this Hive fleet?
12. Could they get ahold of a Genestealer Patriarch specimen and make it broadcast to lure the Tyranids somewhere in particular?
13. Could the path of the Tyranid advance be made to curve by focusing our efforts against one of the flanks of their advance?
14. What opinions do the Eldar, Vanahiem, and Chapter Master Julius have on methods of getting Amir Ka to prepare? This really seems more like a naval and diplomacy problem than a tactics problem.
15. Could we hit and manually sterilize via bombardment the unaffiliated Ork and Chaos worlds outside of Amir Ka and Ironbusta's Domain?
16. What sort of force would we need to take each of the minor worlds out with a minimum of casualties?
17. Would specifically dilapidating and partially sterilizing our own worlds that are likely to be struck first be possible? Something that doens't permenantly affect life bearing capacity after it wears off.
18. Would radiation bombs work for the above purpose? With samples of the ecosystem preserved for when to radiation wears off in a century?
19. Are the Tyranids faster than our ships in Slower Than Light maneuvering?
20. Could you add the number of ships per escort flotilla or cruiser squadron to the FAQ?