Hi labeartique, it is best to spend some time deciding, I will pass on my thoughts as you have numbered them. I think . . .
0) stick with one checkout system and you need an eBay store to use store inventory format e.g. 30 days etc..
1) yes, I prefer generating listings via Auctiva and using all those lovely free options including the scrolling gallery and scheduling.
2) listing insertion fees are payable to eBay anyway, inventory format listing fee is lower than for auction but FVF higher, ideal for slow moving items.
3) Not sure which link you are referring to.
4) Yes
5) Yes
Just remember that an Auctiva store is an alternative way of displaying your eBay listed wares.
Also you do not need an eBay gallery picture to see pictures of your item in an Auctive store display.
I think that is enough for now