1 piece or 2 pieces I have used them on the FM2, FM, FE, F3, F2 without any problem. There must be something wrong with the cap if that is the case. You can buy the 1 piece battery if you want.
I think you can skip the flash meter but a good used one for $50 is nice. For that type of shooting I agree you don't need iTTL or CLS. Just manual is simpler.
For long exposure and exposure on tripod I always use manual mode so the eye piece blind isn't needed. I do feel the Df without the finder blind is a cost saving I don't like.
My 60CT4 is nice because I can use it at ISO 1600 and f/5.6. At that setting even with bounce it will recycle very fast.
What would you want for your 45 with bad battery pack?
I don't generally use the manual mode on the flash with the flash mounted on the camera. I use it for a lighting setup with the flash on stand or tripod in which case I use my Metz 60-CT4 which has 27 steps manual power.
I manual focus with the 45-CT1, 60-CT2 and 60-CT4 without a problem. The 45 series although smaller is a bit heavier because it's self contained. On the 60 most of the weight is on your shoulder.
The 60CT4 is nice because I can use ISO1600 @ f/5.6. At that settings it can recycle very fast.
ISO invarriance depends mostly on the number of bits used to digitize the RAW file. Because RAW files are normally converted to 8 bit per channel JPEG then a 14 bit RAW should have 6 stop range.
For me I would never use LO. If I use lower ISO because it delivers better quality and LO does not and I have lots of ways to reduce light intensity so I don't ever use it. I would use the HI though if I need it.
The reason that you said it doesn't have battery grip. All F4 has a battery grip and depend on which version the grip is different. The F4 doesn't have the vertical grip. The F4s and F4e has the vertical grip. The F4 without the vertical grip originally was not sold in the US.