I've come to believe that the only way any of herbs will be effective (bioavailability issue potential) for me will be to prepare either a decoction, infusion, or tincture.

Most preferable would be a glycerine based tincture, but suspect (or have read) that the superior medium is alcohol. It is not that I don't drink alcohol, I just have an easier time with glycerine based products (less nauseating).

Can anyone give me some 411/input on the following:

1 The preparations listed above: pros and cons, Potency/efficacy comparison.

I discuss the drawbacks of liquid extracts of herbs about a third of the way down on my home page:

http://www.mountainmistbotanicals.com/#FAQs

Powdered herbs have a high surface area, which actually helps with extraction and absorption. If you want to stick to powders and increase the extraction and absorption even more there are tricks to doing this. For example you can take some TMG and yucca root with your herbs. TMG is a very effective solvent and yucca root is rich in saponins, which increase cell wall permeability enhancing the absorption of compounds taken with it. This is why I always add saponin sources in my formulas.

If you still want to go this way it can be done where the strength is the same, or stronger, as the alcohol based tinctures, but it will take a little extra work and equipment.

First thing you want to do is to go ahead and tincture the herbs. When this is complete strain the liquid from the solids. Here is where you need to decide how strong you want to make the final product. Let's say you want a standard 1:1 concentration. Add an equal volume of glycerine to the tincture as the alcohol in it. In other words if you have 100ml of tincture add 100ml glycerine. If you want the final product to be twice as strong add half the volume (50ml) of glycerine. Blend the tincture and glycerine well.  Now the fun part and the where the extra equipment comes in. You will have to pour the mix in to a vacuum flask and seal the top with a stopper. I would set up a condensing coil between the vacuum flask and the collection flask that you are going to apply the vacuum to. The collection flask will need a stopper with two ports, one to connect to the condenser and one to go to the vacuum system, but the tubes through the stopper should not extend more than a few millimeters in to the collection flask to prevent sucking the alcohol up in to the vacuum pump. So when you are set up properly you will have the vacuum flask connected to the condensing coil and the other end of the condensing coil connected to the in port of your collection flask. The out port of this flask connects to a vacuum pump. This would make a lot more sense if I could draw pictures on here. Anyway you need to warm the tincture glycerine mix a little, but not to the point of being hot. The collection flask should sit in a bath of ice to keep the alcohol-water you will be collecting condensed. Now you will be ready to go. Turn on the vacuum pump. The warmer solution will boil from the reduced pressure without reaching the normal boiling point. The alcohol and water will condense in the condensation coil and collect in the second flask. The ice will keep it from boiling off again. When you pull off all the alcohol and water the herbal compounds will be left behind in the glycerine. And you can re-use the alcohol-water mix to tincture more herbs.

If you do not want to go through all the hassle then the next option is to buy powdered extracts. You can either take these straight or blend them in your glycerine or a glycerine-water mix. They are not going to be easy to find though.

3 Can powdered herbs be used to make any or all of the above?

Yes.

4 If yes - while preparing with powdered herbs, any modification or adjustment to instructions? (Most techniques I've read discuss preparation with leaves, flowers, parts/ cut/sifted herbs - not powdered.)

You would use less of the powder for since it takes up less volume.  It is hard to give exact amounts, but I would go roughly 1 tablespoon for every cup of alcohol-water or alcohol depending on the proof of the alcohol you are using.

The other thing to remember is because the powder will compact on the bottom reducing exposed surface area the tincture jar should be shaken up frequently throughout the day to keep the particles more exposed for extraction.

5 Possible to make a Master Brew using a combination of herbs - powders, leaves, flowers, etc. all thrown in the pan/container while extracting?

It can be done, but I don't recommend it for several reasons. First of all different herbs can require different concentrations of alcohol. Most herbs can be extracted with something like 80 proof vodka. More resinous herbs may require 90 proof or higher alcohol. And some herbs such as yerba mansa root require 190 proof (95% alcohol), which by the way is illegal in many states. The second problem is that the higher concentrations of alcohol can damage the active components of some herbs such as echinacea, which has polysaccharides that can be damaged by the alcohol. And finally, if you mix the herbs certain compounds in the herbs can extract faster than others, which can saturate the alcohol too fast to allow proper extraction of the slower extracting compounds.