It all depends upon whose procedures you going to follow?
Let's see if I can figure out how to word so not confusing? So there is probably more than two methods but originally when I was in school I was taught to evaluate the watch make your repairs once the watch is functional then it's Disassembled and cleaned and we go off with assembly lubrication rating etc. The reason for this is once the watch is nice and clean and properly lubricated other than minor regulation if you have to be disassembling the watch or taking the balance wheel out multiple times you will screw up the lubrication. That means if you screw it up you basically have to clean it again and start over.
In modern shops and the modern schools there now teaching pre-cleaning. They do not like to work on dirty watches they like clean watches because they feel they can see things better. So movement assembled is run through a special machine They usually has a shorter cleaning cycle. The other reason it's a separate machine is to keep the cleaning fluid in the final cleaning machine much cleaner. Now you can evaluate the watch do the repairs on a clean watch then it's taken all apart Clean and lubricated minor regulation same as above then.
@Maxppp If you have never serviced a watch before I do recommend you practice on something else first, for example a Vostok 2409. It's a stunning timepiece you have there and there's a real risk you'll damage something or lose a part which might become very difficult to replace. There's always a risk, even when you're experienced but with experience you'll learn to minimize the risks.
Best of luck!
Any way suggest the best way of removing this screw.
Do not want to drill it incase i destroy the thread.
Ain,t there some chemical i can use that will dissolve the screw but not the bridge?
I might misunderstand you, but isn't the normal procedure to first service the watch and then (and during the service) do the fault finding in case the problems remain? It's an honest question, not an opinion.