If a crisis should arise, the government would/will have to take action.

[sitifan]

1. If she should be late, we’ll have to start without him. (NOT* If she should be late, we’d have to start without him.) [my bold, Practical English Usage, 2nd edition, section 261]
2. If a crisis should arise, the government would have to take action. [my bold, Leech & Svartvik’s Communicative Grammar of English (section 277)]
The modal verb "would" cannot be used in the main clause of #1, why can it be used in #2?
Can the modal verb "will" be used in the main clause of #2?

 

[5jj]

1. If she should be late, we’ll have to start without him. (NOT* If she should be late, we’d have to start without him.) [my bold, Practical English Usage, section 261]

Swan actually wrote "Would is not common in the main clause `in these structures">

Can the modal verb "will" be used in the main clause of #2?

Yes.