good point, I have done a few straight brett beers, but usually bacteria is going to be involved. That being said whether you are using sacc or brett plastic equipment should be routinely replaced.
I would go for it. I go from a slant to a 250 ml starter then jump to a 500 ml starter then pitch in a 5 gal batch OG < 1.060
You have much more yeast in one of those vials than a slant.
If you are going to batch sparge on higher OG beers, you will need another kettle. Once you have another kettle, HLT, you might as well batch sparge all the beers you make. If you are reluctant to buy another kettle you use more grain to account for inefficiency
I have seen this idea proposed before. The only concern I have is you will be sparging your grains with wort, to me that seems inefficient. Trying to rinse sugar off grains with wort? I have never tried this though. I recommend the BK and HLT be separate.
Good advise, also i would not purchase another kit form them. I have not purchased a kit in a while, but there are companies that disclose all ingredients.
for smaller amounts I use plastic tupperware, may not be the solution for such a large amount. I think any plastic container that has a rubber seal in the lid would work stored in a cool area