Any way to opt out of whiteboard interviews without seeming difficult?
Offer a substitute rather than a refusal: a take-home you have already done, a walkthrough of public work, or pair debugging on their codebase. Companies that decline all three have told you something useful about how they work.
How do you estimate a task with unknown parts?
Timebox the unknown separately. Estimate the known work normally, then add an explicit investigation slot with a decision point at the end. Merging the two produces a number that is wrong in a way nobody can inspect.
One offer has more cash, the other more equity. How do I compare them?
Value the equity at zero and see whether the decision changes. If the lower-cash offer still wins on work and people, the equity is upside. If it only wins with the equity counted at face value, you are being paid in a lottery ticket priced by the seller.