Yashica Electro 35 Gx Battery help !

First off, there's a few places to reference with some collective wisdom on what has worked for others. Karen Nakamura's site has a comprehensive page that covers most of the discontinued battery products - Mercury and otherwise -  and what can be considered as replacements for them. It has most of what's known to work on one page... www.photoethnography.com/ClassicCameras/batteries.html

There are 2 pages from Mike Butkus' site that deal with a number of battery issues that refer to multiple solutions found over the last few decades... www.butkus.org/chinon/camera_batteries.htm and www.butkus.org/chinon/mercury_replacement_batteries.html

There might not be a definitive answer to cover every individual camera - even the same model can vary in how it handles a given solution, since chemistry and age are never constant or 100% consistent. The best advice I can offer is to assume that whatever option there is will end up being a compromise of one type or another, and there may be procedural adjustments (like, changing ISO values), higher costs and/or more frequent replacements above what the original battery would call for, or additional purchases (1 or 2 adapters) that may be needed to get some use of the camera/flash/winder at some diminished level that won't match the original's performance, but will be close enough to extend the gear for continued use.

My experiences...

• Zinc Air alternatives (with or without #9 O-Rings as space adapters) will work, but will not last long. Once you peel off the 'sticker' to activate them, it's a constantly running engine that will never go back to idle mode.
  
• Adapter's with built-in voltage circuits work, but they can be expensive to purchase as up-front costs will be higher than a bag-o-batteries, so it becomes a question of how much cash do you want to throw at a solution over the lifetime of your use for what it/they will power.
  
• SR44's (Silver Oxide) are better alternatives than LR44's (Alkaline) at the same voltage and will be more accurate over a longer amount of time - and yes, their power curve is not as 'flat' as any Mercury cell it would replace, and will eventually mismatch what should be a correct reading. The answer to that is to occasionally check your readings with an alternate meter you trust, to judge reading accuracy and then change out cells when they start to fade.
• If the PX-640 work-alike you're referring to are the Wein Cells or those from Excell - yes they're expensive, but not too difficult to find. Amazon usually carries one or the other, or both - at least in the US. Again, it's a case of your expense over the lifetime of the gear, and if it's worth it - to you.

If the camera requires a battery for metering and shutter actuation, then yes the wind lever can 'freeze' until the battery is replaced. Contax and Yashica cameras that share the same electronic/magnetic shutter circuits all act this way, as do many other maker's cameras that use the same type of shutter systems. FX-D's, 139Q's, RTS, and several others will require a 2nd set of batteries on-hand to make sure you'll keep shooting when old ones fail. Yashica FX-3's use mechanical shutters, which sets them apart, but they are fully manual cameras too, so if someone prefers some programmed automation they're out of luck.