I owned the Sony Zeiss 50 f1.4 and liked it quite a bit. It was sharp (though not as sharp as some of the most recent super sharp lenses), it focused very quickly, fully supported the AF points all over the a99ii's screen and rendered beautifully. I purchased my copy used for around $900 four or five years ago.
I like 85mm for portraits best (even for full length, but it takes a lot of distance). However, if you have a 50, the 50 can work very well. Personally, I wouldn't purchase a 35mm primarily for portraits unless I was shooting in a location that I didn't have enough room for something longer to work. However, many like the wider angle, but I prefer more telephoto.
Outdoors in good light I would think either would be fine. Indoors I don't think either would work.
Interesting. I've used mine extensively with a 70-400 and occasionally with a 200-600.
Most of what I've read and experienced suggests that teleconverter performance can vary widely by manufacturer, generation, which lens it's mounted on, etc. For instance, the Sony A mount 1.4 TC worked pretty well, but the consensus was that cropping typically produced better results than the 2.0 TC, although the 2.0 worked better on the 600f4. In Sony's E mount the 1.4 and 2.0 TCs work very well with the lenses that'll mount to them, but even here, the 2.0 TC is generally commented to not work as well on the 70-200f2.8 than it does on the 600f4 and 100-400. I believe there are similar combinations that work better and worse on the Canon and Nikon lenses as well.
A custom build desktop is probably the least expensive, highest performing way forward. I've been running CAD systems for years and using Lenovo laptops to do it so I typically purchase them used for my editing. I'd get something like a P50 or newer which can be had for $600 or less with an i7, 16+GB RAM, SSD, and nVidia Quadro graphics. I've owned or helped others purchase five or six of these and they just work. Additionally, all of the parts are available for even 9 or 10 year old systems so their easy to upgrade when necessary. One not often talked about item I particularly like about my P51 is that it has 3 hard drive bays.
I've occasionally used generic batteries. On some cameras it seemed to make no difference; on others it seemed to create small issues which I wouldn't thing would be related to the battery, but it only happened with the generics installed. I've never studied the difference exhaustively, but, depending on the generic brand, there are sometimes some subtle differences. Since the time I saw the small issues I have chosen to stay with Sony branded batteries. I did purchase a couple of used branded batteries from KEH once which I've had good success with.
I have also purchased a couple of ebooks from Gary. He's good and the books are huge and thorough and easy enough to follow. He also has some interesting youtube videos about the Sony stuff.
I don't own Nikon stuff and I don't own an a7riv, but I do have an a9ii and I have done quite a bit of reading on AF performance. Much of what I've read suggests that F mount lenses do not adapt nearly as well as Canon's EF mount to Sony E-mount. Much of this information is, perhaps, a year old, so maybe an adapter has been released in this time which works better, but do your research thoroughly.
As to Sony's AF performance, what I've both seen and read is that what is posted about being ~2 years ahead of the others seems common and reliable. That said, cameras made in the last 4 to 6 years tend to have very good AF. The way I take this is that, while the Sony AF may be slightly better, the difference tend to only come through at the extremes of performance, rather than the general use cases where it's hard to get a camera which will not perform well nowadays.
Finally, while it's certainly true that adapting from one brands lenses to that same brand's mirrorless will perform better than across manufacturers, even adapting EF to R or F to Z is still adapting.
As simple as it sounds, if you have not yet, shut the computer down and then start it back up.
The OP has been given some excellent references in Mark Smith, Artie Morris, etc. These guys shoot the lens a lot. Mark in particular typically shoots it handheld. There are a lot of great images with this lens. Check out Fred Miranda as there is a big, long thread of 600mm shots. Honestly, I'm blown away with what the 600 will do with a 2xTC on it.
One way to attack the printer ink cost is to use 3rd party. I switched my Pro-100 to Precision Colors. I love the output and I can refill a set of cartridges for very little money. The ink isn't as fade proof so I only print stuff for myself, but it works pretty well.