I'm brand new here, and to intentionally gaining, but maybe I could help answer Qs 3 and 4 as I thought about it. Because I just made this decision from a similar point.
For background, I have been lean and very athletic most of my life, but did not lose all my pregnancy weight and then put on about another 35 pounds due to overeating and prescribed medicine. I kept this weight on for 3 years. As the medicine was switched, I lost some weight, reduced my BMI, and began to look a bit more like my old self.
I had intended to get in shape (been inactive this year due to the pandemic and oppressive heat records here), start doing sports again, and lose weight, trying to regain my old physique. But then I found out that gaining more subcutaneous fat (soft and actually protective of health -- see healthy obesity) while losing some visceral fat (the unhealthy kind) could actually be a healthier option for me at this point in my life. Add to that my fascination with feederism which I discovered a few months ago, and my wanting a curvier shape all my life, with much bigger breasts and butt. So I decided to gain first before losing some later to lose some of my visceral fat. It's easier to put on more subcutaneous fat first when heavier, and then that is harder to lose when losing and the visceral fat burns off more easily, so I should be able to retain it. Subcutaneous type of fat tends to distribute more evenly, so I should pad the areas I want to. Then, after I hit my reduced visceral fat goals and get back in reasonable physical/exercise shape, I'll then gain again until I get to my desired shape. I hope to reach a level of physical activity and body shape that I love, kind of steering and correcting towards my personal goals.
So I took the plunge and started gaining, and added a heavy cream program. In even just the first two weeks, I felt tons better, had more energy, was happier, and had silky soft fat starting to pad my body. For the first time I could feel it jiggle as I walk. I am so pleased with the results, I am renewing my commitment to gaining a substantial amount to pack on the subcutaneous fat over the next 4 months. I actually think this may be one of the best health decisions I have made. And it sure is sexier to me.
As for matching with your fiance, obviously your and her personal preferences and preferences about how each other look are key. But if you gain some and she continues to try to get in shape and maybe lose some, your respective physical exercise levels and weights will match better. From what you say, she may feel better about that.
It's an important decision whether to gain or lose, because I hear many stories of people moving past a certain point (maybe 20-30 pounds more gained relatively quickly) and then not being able to take the weight off later, or at least not at all easily.
I looked at many posts on here about subcutaneousvs visceral fat and how to gain healthy, so you can find info here or look into "medically healthy obesity", which seems to be the medical research term.
Good luck on whatever you decide!
3 years