Tony's mystery method?........Easy Off Oven Cleaner to remove tarnish. I have used it many times on OR and officer wappen. You must spray it on over a sink wearing a rubber glove on the hand holding the wappen. Care must be taken.... spray and use your soft tooth brush to work the surface. Do this for 60 seconds or so then wash off the chemical and observe your results. Repeat as desired until you get the results you want. Watch gilded areas to make sure they are not being removed and be extra careful on officer gilded wappen. On this Waterloo officer wappen, I would not use it for fear of removing the black paint in the letters. Sometimes, the brass turns a sort of pink colour due to the chemical reaction, I always finish off with Hagerty Silver Foam paste again using water and a soft tooth brush to remove this. The paste builds up into a brown foam when mixed with water on your tooth brush. It gets down into the fine details of the wappen.
Basically, I use the oven cleaner to get heavy tarnish off then go at it with the Hagerty paste. If the tarnish is relatively "light" I just use the Hagerty.
Regarding spikes, spines and other fittings.....you could begin with Brasso to remove heavy tarnish and then finish using a cloth polishing wheel with jewelers rouge. I use the wheel and rouge to clean chin scales, but you must be careful how you apply the scales to the spinning polishing wheel. You can also use a dremel tool with polishing bit and rouge for tight spots. Cleaning chin scales takes a lot of time and hand work. You can never really get under each scale to completely clean the one it overlaps. However, brown tarnished brass still looks a lot better when cleaned up. As a last comment..... this tarnish has taken 100+ years to build up. I would recommend cleaning a piece only once while you own it. Excessive repeated cleaning will damage the fittings. If you can live with the level of tarnish that is on your fittings, then leave them alone. Just as with the slight depression in Marcels' great Waterloo helme, it wasn't major so leave it alone. It is quite hard to wet and block small dips like this in a helmet. You may go through the entire process and the depression comes right back. Worst case scenario, some finish flakes off the area!
Regarding restitching, there is only one restitch in a helmet because of the delicacy of the small bits of leather between the stitch holes. These take a beating when you clean out the old thread and again when you restitch with new thread. Friction is your enemy here!!
One last thing......for Gods sake, DO NOT glue visors back on! I have run into this several times I believe done mostly by older collectors back in the early days of collecting. This glue mess can be removed and fixed but takes time. The glue trick can be hard to spot so check your visors gentlemen! Here is what to look for.....on the outside look along the stitch line and make sure that you see thread in every hole. If thread is missing from large sections then what is holding the visor on? Excessive black shoe polish applied along the stitch line is also a red flag. Lastly, flip the helmet over and examine the area where the visor meets the shell. Pull the shell back from the visor gently and look for dried glue. Sometimes you will see threads of glue between the shell and visor. Smell the area, if recently glued then you will smell the glue! Last, when in doubt, leave it alone or spend the money and send it to someone like me who has worked on dozens of different helmets. Do not take it down to your local shoe repair guy! Do not screw up a piece of History trying to save a few bucks! Here endeth the lesson and I will now climb down from my soap box.