There are several misconceptions here. There can be site finished solid wood or site finished engineered wood. Having it finished on site is the variable. The two are not mutually exclusive. Foundation type often demands engineered wood, and that does not mean accepting a lesser quality material. As long as an adequate budget is in the picture, quality in either type is achievable. Site finished has everything to do with finding the right artisan. Prefinished has everything to do with the company that creates the product. It is a shift in emphasis for responsibility of a successful install. A NWFA accredited installer and installation for site finished is likely to be several notches in expense above the install of a medium quality prefinished product. Choosing to use a lesser skilled finisher simply because of budget, will likely have results that you are not happy with in the long run. Engineered wood, if it's a good quality, is indistinguishable from solid wood to the naked eye. IF it's installed as a Permanently affixed nail down or glue down. Floating floors are a whole other category of installation and often perform unsatisfactorily because of improper site prep and low quality sound pad. That is regardless of the material being floated, including laminate. Any floor that is not flat enough, and experiences bounce, is not flat enough for Any floor but carpet to be installed. Deflection though, often has more to do with the joists being inadequately specced to the lowest allowable by code rather than the subfloor thickness. When hard floors are desired, the home has to be engineered to accept them from the design phase. This is one area that being "code compliant" results in too many problems down the road because the code is inadequate for the construction level that most homes now wish to have. The use of a good quality vacuum is a recommended care routine in addition to dry dusting. If there are no other issues in the home, such as inadequate kitchen ventilation, a vacuum will remove dust from any micro bevels in a prefinished floor---or the inevitable micro gaps in a site finished floor.

I doubt if you will be happy with walnut, the softest of hardwoods. There is no finish that will make it much harder. It is what it is. The opposite end of the hardness scale is the ipe you mentioned. If you want a similar grain you may consider red birch, a beautiful light cherry colored wood much harder than walnut. Birch, as with all close grained woods, does show the indentations it gets more readily than the more textured woods like oak. This is because the smoother surface makes them easier to detect. No finish is perfect. Aluminum oxide is slow to wear through but also harder to repair. We have in our house a lifetime guaranteed wear through finish (Trustor WearMax) but I would not use it again. No sign of wear through but we do have indentations etc that I can not touch up to my satisfaction and I'm an expert at that type of thing. I suggest you walk through this with a competent professional to find a combination of ascetics and durability to suit your needs.

If you want the STRONG GREY colours shown in your photos, you will need to work with factory finished. If you like the liming or cerusing effect (the white in the grain) then you will need to work with the factory finish. I've heard of a small handful of HIGH-END professionals who can achieve these looks on a site-finished floor. If you think that the population of the US+Canada is around 375,000,000 that is a SMALL amount of humans....like 100-150 professionals who can handle a job like that. Imagine the price tag! We will see this level of workmanship on $25Mill homes. The price of the professional is not at issue in a home that has multiple millions of dollars in financial backing. I understand your Project Manager's concerns. Repairing damage to a factory finished floor isn't fun...but it isn't difficult either. The worst case scenario = cut out the plank and drop another one in. A bit of glue...a bit of waiting and everyone's happy. I'm not sure why the push back is so hard (the PM has probably had something go wrong in the past and had it bite him/her in the arse pretty hard). So long as you assure him/her that you will pay for all the damage control (ram board, etc) you should be fine.

Figure out which finish you want to use, then follow manufacturer's sanding guidelines. Generally, you will start with 36 grit ceramics and work your way up to 120 grit silicone carbide. Some manufacturers requires 150 grit for a final pass. It all depends on the manufacturer's guidelines.