I would also recommend the coach.

If you travel by train you'll be arriving at Euston in the heat of the rush hour.

Based on what you say, I'd avoid the Piccadilly Line, since getting to it at Kings Cross will not be particularly easy and there will inevitably be some steps. However, if you do take the Piccadilly Line, although you might well have to stand for the first part of the journey, I should think you'd get a seat easily enough after the train has passed through central London and is on its way west.

Taking a taxi to Paddington will not involve any steps, but the trip might be quite slow (considering the short distance) because of the traffic, and then you have to find the right train and so on and so on.

Taken all in all, I think the coach sounds the most sensible option.