We wanted floor outlets but have a slab on grade so they had to be allowed for ahead of time. When it came time for the wood floors he had them move two outlets about a foot by running wires under the hardwood. Turns out the original locations would have been better. Further, we ended up with wall plates on the floor which were never designed to be stepped on or have furniture legs on them. The original locations would have had nice recessed brass outlets. Bummer. The best thing to do is wait until the furniture is place then install them if there is good access from beneath. Basements must be nice.....

Do not butt the floating floor to the nailed hardwood, leave an expansion gap between the two floors at the doorway and cover the gap with a T-moulding.

If the manufacturer allows nail/staple down installation of their engineered floor, that should be sufficient. Since your selection is NOT wider than 7.5", the warranty is valid. They won't address any issues with movement or squeaking though. I can imagine there may be a little more sound emanating from the floor but that would be acceptable to me, especially if one considers what would have to be done if your floor was damaged and required removal after being glued and nailed. If I was going to glue the floor down with a full spread adhesive, I see no advantage to nail/staple it also. For a better underlayment I would recommend Aquabar B, which is carried by Home Depot. https://www.homedepot.com/p/Fortifiber-500-sq-ft-36-in-Wide-x-167-ft-Long-x-7-mil-Thick-Aquabar-B-Tile-Underlayment-Roll-70-195/202592567?mtc=SEM-VF-F_D23-G-D23-23_6_FLOOR_AND_WALL_TILE-Multi-NA-NA-DSA-2150536-WF-New_Engen&cm_mmc=SEM-VF-F_D23-G-D23-23_6_FLOOR_AND_WALL_TILE-Multi-NA-NA-DSA-2150536-WF-New_Engen-71700000077168191-58700006540356281-39700059170875303&gclid=Cj0KCQjwhr2FBhDbARIsACjwLo33OGF8tiZIs8wUOKWMZ1XxG1JDAIsLbwsMH82Ahl0cZp9ethZdDe4aAsqZEALw_wcB&gclsrc=aw.ds#!bvstate=ct:r/pg:2/st:p/id:202592567

Cabinets are permanently installed built in obstacles. All floating floors should go around built ins, with an expansion gap, not under them. Only permanently fastened floors like wood or tile go under cabinets. The cabinets themselves need to be shimmed to the finished floor height, as that finished floor is the basis of all room measurements.