Flux- pine sawdust and candlewax. Stir it in and let the dross sit on the surface to help cut off any oxygen getting to the melt.
Recharging the pot- if not in a hurry, I put cold ingots in CAREFULLY and slowly with tongs. I can see how some want to warm the ingots to get rid of any possible moisture since water and molten lead can cause visits from the tinsel fairy. If I'm in a hurry, I'll hold the fresh ingot over the pot with tongs and melt it into the pot with my propane torch. This gets the pot back to temp much faster. After casting, I put lead into the pot to bring it up to full and shut off rather than let it reach casting temp so I'm with the side that leaves lead in the pot.
As for bottom fill out- I have no issues when using a bottom pour IF the mold is hot and the lead is at 800+. I tilt the mold slightly and hold it in that position till it fills and them leave a generous amount of lead on top of the sprue plate. That insures as the lead cools, it won't make cavities and helps to keep the mold at operating temp. Most of the bottom fill out problems I've encountered were solved by making sure the core pin was at operating temp. A cold core pin will lead to poor fill out in the base and cavities in the center of the minie. When using the ladle, you'll have the mold to the spout at an angle as you start to pour, it's essentially the same as I do by tilting the mold on a bottom pour pot. I also get the mold up close to temp with a propane torch. No need to roast the mold, just leave the halves slightly separated and play the flame across making sure to get some in between the halves to the core pin. After a bit of that, then start casting, again, leaving a generous amount on the sprue plate to help get to operating temp. Toss the first 10 or so and with this method, you'll be in production pretty quickly. I put the sprues right back in the pot as they're knocked off. Some don't do it this way, I've never had an issue. The sprue is close to molten already and I run a 20lb pot so that little bit of lead won't affect how hot my pot actually runs and I do monitor it with a thermometer. Incidentally, I've noticed that the numbers on the Lee pot track about with 100F increments so "8" on the pot control is about 800F. I leave mine between 8 and 9 while casting minies.
When doing a production run, I'll cast until the pot gets below half full then take a break and recharge the pot. I'll leave the mold propped on the edge to keep it warm as the new lead comes back to temp. Again, it won't be quite to operating temp and you'll have to toss the first 10 or so but it will quickly be back in action. When things are going well, I'll cast about 80-90 minies per hour this way.