NiSi PRO Nano IRND Filter (77mm, 10-Stop)

No, the NiSi 77mm PRO Nano IRND 3.0 Filter (10-Stop) would not provide enough coverage for solar eclipse photography. You need at least 16 stop ND, preferably with UV/IR cut. Nisi has the following option. https://bhpho.to/3vaPO0g

No, the NiSi 82mm PRO Nano IRND 3.0 Filter (10-Stop) would not be compatible for your usage needs. It does the opposite of what you are looking for; it is designed to block IR to the extent that there will not be infrared pollution when taking a long exposure. As an alternative, the filters in the NiSi Swift FS ND Filter Kit (82mm), B&H # NIRFSNDSWK82, are designed to pass all wavelengths of light evenly, so this would also be an alternative for your usage needs. Also, as standard neutral density filters pass along infrared light unless it specifically indicates it has IR reduction, a filter such as the Hoya ProND EX 1000 Filter (82mm, 10-Stop), B&H # HOPNDEX1T82, should also work for your usage needs. https://bhpho.to/3ieQ30o https://bhpho.to/3LZm8bn

Typically this is determined by the lens in question, but because the front and back are the same diameter, it shouldn't be an issue.

While the NiSi 72mm PRO Nano IRND 0.9 Filter (3-Stop) does have a front thread, it is not advisable to use two variable ND filters at the same time as it would create undesirable results.

Yes.