Hello all.
I got some questions regarding sub-panel replacement but first a background. Sorry if is a bit long. House build in 1947, mixed of old wiring (no ground, cloth type insulation) with some romex and THNN wires. Original service panel is from Federal Pacific. Sometime in 1980s house got remodeled and addition was added and a new service with meter panel was added outside of the house (Eaton style main breaker 100A..GE brand??) The original FP panel became a subpanel inside the house. The FP panel is being fed by 30A breaker from the outside panel. Looking at the wires at the outside panel it seems that these are 8 AWG wires which seem ok for the 30A breaker. EMT conduit attached outside wall with approx 25ft length before entering in the house by some type of 6 x 6 Jbox in the attic.
Since the issues of FP panels and the mess with illegal wirings, I would like to replace it first then change all the wiring through the whole house to be per the current code. Planning to pull homeowner permit and do the wiring myself.

My plan is to get a 100A Eaton CH or Murray panel to replace the FP subpanel. The sub-panel breaker at the service would be 70 or 60A. The original feeding conduit will be abandoned or removed. Instead of going up and sideways along the wall, it is planned to start from the bottom of the service panel for about 4 feet then go inside the house and reach the subpanel through the craw space.
Here my questions:

1- Is 6 AWG wires enough for 70 or 60A or should use 4 AWG? Not sure at this point. Estimated run lengh of 20-25 feet.
2- If I understood, romex NM wire not allowed in conduit. THNN or similar only. Romex wire ok on craw space by itself and properly attached. How can I go from wires in a conduit (outside 4 feet ) then go inside the house in the craw space without a conduit? If this is not allowed, do you recommend running the wires with grey PVC conduit from panel to panel?
3- Is 1 conduit ok for 6 or 4 AWG ie. 3 wires plus bare ground?
Thanks in advance.