If someone utters (2b), Japanese people will understand "開いている" means "営業している(doing business)," so (2b) is the possible counterpart of (1).
It may be helpful to associate "開" with "open": something is open to you, and as a result, you can gain access to the service that "that something (a store, shop, restaurant and the like)" provides.
On the other hand, it could be useful to connect "空" with "empty" in mind: there is an empty space in something, and therefore, "that something" can accomodate someone or something else.
You may hear (3a) when someone is asking about your schedule, meaning "Are you free tomorrow?" You could also hear it when someone is making a reservation:
b. Do you have an X available for tomorrow?
If you replace "an X" in (3b) with "a room <hotel>," "a table <restaurant>," or "a ticket <theater/plane>," the resulting sentence might work as the counterpart of (3a).
When you read "空いて" as "すいて," however, the idea is rather closer to the idea of sparsity.
(4) a.もう空(す)いたの?(Are you hungry already?)
b. 今日の首都高(しゅと+こう=首都+高速道路)は比較的空(す)いている. (Today, traffic on the Tokyo Metropolitan Expressway is relatively light.)
c. 今日はレストランが空(す)いている. (Today, the restaurant is sparsely populated.)
The intended meaning of "空いている" in (ia) is actually "free" or "vacant."
(i) a. この席は空(あ)いていますか?
b. Is this seat taken?