Yevamos 41b - Chalitzah within 3 Months

The gemara says that based on the rule of kol ha'oleh l'yibum..., chaltizah cannot be done so long as there is an issur to do yibum (provided that the issur will go away sometime in the future so the issue is only about delaying the chalitzah - rashi). The rationale seems to be that the chalitzah is considered "weak" whenever yibum isn't possible (that is the implication of rashi that when you can wait, you should wait to have the chalitza done b'heter). The gemara asks that we find that chalitzah can be done to a women who is a safek and may be the sister of his zekukah, even though it would be assur to do yibum to her until the safek is clarified. The gemara answers that when the issur of yibum is bec. of a safek, if Eliyahu Hanavi would clarify the safek then yibum can be done, therefore we consider her eligible for chaltiza. But within the first 3 months even if Eliyahu Hanavi would determine that she is not pregnant we would still not allow yibum since there was a gezeira of havchana, not just a chashash of her being pregnant (as we see that even by a ketana and many other women who cannot possibly be pregnant are included in the gezeira of havchana).

We see that even if practically yibum cannot be done since the safek has not been resolved by eliyahu, we would still allow chalitzah. Why? Shouldn't we say that since practically speaking yibum can't be done, so chalitzah can't be done either? It seems that the reason why chazal instituted an issur of chalitzah when yibum can't be done is not bec. the inability to perform yibum makes the chalitzah weak, but rather bec. people will see that chalitzah was done in such a situation and come to do yibum in the same situation. Therefore, in a case of safek they were not concerned bec. people who see that chalitzah was done will assume that the safek has been resolved and that is why chalitzah was allowed to be done. But in a situation where there is an issur yibum bec. of a gezeira i.e. havchana, if we would allow chaltiza, people will assume that if chalitza is allowed then yibum is also allowed.