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.
How much notice is reasonable when the handover is complex?
Whatever the contract says, plus a written handover that outlives you: runbooks, access list, the three things that break at 3am. Extending the notice period rarely helps; writing things down always does.
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.
Any downside worth knowing before I commit?
It commits you to a format that is tedious to migrate away from later. The first weeks also look worse than doing nothing, which is when most people abandon it.