I haven't really visited during peak season. Still I think there isn't likely to be a problem since there is going to be a bigger crowd at space needle and chihuly glass museum which is located at Seattle Centre and is a bigger tourist attraction. Still there is no harm in booking ahead to save time.
There is no selling out of tickets, however there is a capacity limit that is usually only an issue around holidays like the fourth of July and New year's Eve when there fireworks and around the time the blue angles and sea fair come to town
No. The lines at pretty short, if any.
They usually do not sell out. But if you want to avoid a possible line, you can buy in advance.
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