Hi and welcome!
A lot a different questions ... I try to pick some up.
When using the DT50 with a POD HD the standard way to configure is running the rig OVER channel A. Channel B is not need with a POD HD. You can do everything you can do over channel A. This is the default usage. Channel B is for stand alone usage (without a POD HD) and a footswitch.
Some new users try to use this channel B as well with a POD HD. This is misleading too me and soon gets complicated and it is also limited what you can configure in your signal chain. For example all the effect blocks are only before the pre amp section and not behind when used channel B. So my recommendation is: DO NOT FOCUS ON CHANNEL B when used with POD HD. Setup your rig via channel A - the default setup. (And there is also no need to use a separate footswtch). There are some user which have integrated the channel B in their rig, but you should not start with that.
Use the pre amps from the POD HD to set the pre amp section of your tone. It is up to you how to use the reverb. Some use the reverb on the DT50 to save a slot in the signal chain. I do not run out with my slots. So I use the reverb from the POD HD.
Start with 3-4 basic presets like: clean, crunch, distorted, lead and only one amp and assign them to the POD HD. EQ- these basic presets with your amp EQ knobs to your taste. Once that is fine for you, add some effects in the signal chain.
Then you can build up from that. Think about what amps do you really need. Do not choose too much. The same applies to effects. The Line6 gear is great, but it can be a trap with all the possibilities and you may find yourself too quickly in a lot of complexity which you should avoid when starting. (This may sound a bit abstract and not from experience, but I have gone through all of that).
Switching your tones via the fotswitch over the POD HD. Switching presets with a complete different signal chain (amp blocks, effects, etc.) has a little delay. But this has never been a real issue for me when playing live.
I have always found good workarounds for that. When you choose the ABCD FS-Mode then you will cut for example the delay between two presets. That is something you to deal with. With the FS 5-8 mode the delay trails will be kept.
Clean sounds can been very smoothly morphed into distorted sounds via the expression pedal within the same preset: you can assign different parameter to toes and heel position. I use 2 pedals to switch effects and amp parameters. Works great for live performance.
I guess a lot of questions will come up at a later stage as well Ask them here, if you do not find already some answers in existing posts.
Good luck - hope that helps initially.
Gear: Warmoth (H-S-H)-built, JTV 59, JTV 69, JTV 89F, Godin Seagull, Gibson Les Paul Deluxe, POD/HD500, DT50/212, DT50/Head, Marshall JVMC212, Radial JDX Reactor
website: http://www.soundclick.com/wolbai Band: http://www.jump-backtorock.de/