So, consider my advice of "limited experience", that said I've been doing this for a year now without issues and good growth.
Realistically, the answer to most of your questions is "it depends"...
Which method is the best?
Are you asking about EI vs PPS Pro or dry vs liquid dosing?
EI vs PPS pro "it depends" on your growth rate and water change schedules.
dry vs liquid dosing is covered in the next question.
Should the dry fertz be mixed into solutions?
For small tanks, dry dosing fertilizers is impractical, as nobody makes a 1/256th teaspoon measure.. Mixing solutions is a great way to avoid doses so small that they are impossible to measure accurately. Instead of measuring 1/256th of a teaspoon, you can mix 1 teaspoon into 256ml of water, and dose 1ml at a time..
Larger tanks sometimes use liquid mixes for various reasons, mostly to allow the use of peristaltic dosing pumps to add fertilizers automatically.
Should the macro's be mixed into 1 solution or separate?
Mixing them all together is convenient if you know exactly what your tank needs. If you are still experimenting and tweaking dosing, it is better to have them separated.
Personally I find my NO3/PO4 tend to track pretty closely together with my fish load. K2SO4 is something I dose to make up for lost K when I have to cut back my KNO3/KH2PO4 dosing because my livebearers have overpopulated (again). While I currently keep everything in separate solutions, I have considered making a unified KNO3/KH2PO4 mix..
Really the only thing to avoid is mixing anything containing PO4 with anything containing Fe (iron) in the same concetrate bottle.. These two will react to make an insoluble precipitate..
Is adding SeaChem Excel to the Plantex Solution really a good idea?
Yes, unless you mix so little you use it in a week. Plantex CSM+B mixed in water grows globs of mold pretty quickly, as does DTPA Fe.
Is it perfectly fine to keep all of the fertz Dry and dose that way?
Dry dosing is just fine as long as your tank is big enough that you're not measuring impossibly small amounts.. as above, dry dosing doesn't work for smaller tanks..