2X teleconverters generally cost two f stops in exposure. Given the same shutter speed, the depth of field will be reduced, perhaps so that the entire bird will not be in sharpest focus. Other than that, the sharpness of detail would be a function of the quality of the lens/converter. I see no reason to not use the teleconverter to get closer to the bird.
While I don't have the same gear you cite, I have used teleconverters with my Nikon camera/lens with no noticeable degradation of quality. The only factor I've had to consider is a one or two Fstop loss of exposure with resultant changes to shutter speed/aperture/ISO settings.
By all means, get them developed. Expect significant degradation of image quality/color but there is a strong likelihood that images will be recognizable and you might be pleased with the memories. I like the above suggestion to do a develop only to see what you have before you pay for printing the 120's.
You may be new to the "HOG" but your pictures say you are a knowledgeable photographer. Welcome to the forum.
A bargain price for a high end camera such as the D800 is a good choice for anyone who has some experience with cameras in general. Even if your niece doesn't use many of the features available right away, she can shoot in an automatic mode until she learns how to use the additional features. Since she has demonstrated an interest in photography, I suspect learning the use and capabilities of such a camera will give her a great deal of enjoyment.
It is rather late for seeing many eagles. Normal high season is mid December through early March. Either end of those months the eagles tend to find food in other environments and their presence ranges from a few to none at all.
I have that lens and I like it. It is heavy, making hand held problematic at full extension. I usually try to use a tripod or monopod with it. It does as good a job on the quality of image as any other of the lenses I use. Great for birding or other wildlife pics when you can't (or don't want to) get too close.
I have a D850 and do similar pics to what you describe. I would not invest in an upgrade as the D850 does everything I want and more. In fact, my old D300 does most of what I want so the 850 itself was as much a GAS attack as a desire for improvement.
Whatever you decide, have fun with it!
My first digital camera was a Nikon Coolpix 3100 in the early 2000's.
Personally, I enjoy both picture taking and post processing equally. Creating the photograph brings creativity in the composition, exposure and the various technical skills involved. Post processing refines the resulting picture to become what you want it to be. Sometimes all you need to do post is the crop down to your desired image, other times you can do a lot more to produce what your mind imagined when you took the initial picture.
I confess that I am a bit of a computer geek so the tediousness of post doesn't bother me and I enjoy the challenge of what could be. To each his/her own.
A golf glove worn under gloves with finger flip outs might help. The golf glove will give you reasonable tactile feel while you shoot and reduce heat loss.
Finding a local camera club and getting involved might be the best way to get to the next level (or levels) in your photography. Fellow photogs are usually eager to share what they know/understand with others. In this environment you might even help someone else by talking about what you know and do.
Best of luck and happy shooting!
Good advice above and I agree with your view of what constitutes architectural detail. When I enter photo contests, I try to find out who is doing the judging and checking their background, that give me hints as to what they will be looking for.
The best learning experience I've had (beyond trial and error) is finding a local camera club and participate in the meetings. Regardless of the meeting format, most photographers are willing and eager to share what they have found to be useful in their particular field and what not to do. I have found that one can pick up most of the "rules" this way and how to use the various tools available with modern cameras. Just my two cents worth.