Well, it seems difficult to answer, but let me try anyway. I assume you already know the meaning of 「～さえ～ば」, so let's focus on the difference between 「薬さえ飲めば」and「薬を飲んでさえいれば」. Actually this is not a matter of 「～さえ～ば」but of 「－ます or ーています」. -ます 　1.　regularly do 2. will do (strong will or assured schedule in future) -ています 　1. present progressive(action in continue) 2. regularly do 3. remaining result of past action(current state made by past action) 「飲めば」is 飲みます＋ば, and 「飲んでいれば」is 飲んでいます＋ば. Suppose Mr. Y is suffering a disease X. The doctor have some medicine for it. Case I. The doctor says, 「この薬さえ飲めば治りますよ。」＝If only you take this medicine, your disease will be cured. So, the disease X will be cured after he took the action to take it once(or as doctor instructed). Case II, The doctor says, 「この薬さえ飲んでいれば、病気Xは進行しません/そのうち治ります。」=If only you are taking this medicine, the disease will not get worse/ will be cured someday. So, Mr. Y needs to keep taking the medicine for a while. While he is in the state of "taking the medicine", the disease will stay same or may be cured gradually. But not at once. Case III The doctor says, 「2年前にこの薬を飲んでさえいれば、病気にならなかったのに・・・。」=If only you had taken this medicine two years ago, you didn't get the disease. Mr. Y had a chance to take the medicine before, but he missed it. The current state is "have not taken the medicine." Hope my answer will help you!

Umm..., difficult. "すれば" = if "する" = now not "する" = now "しない" The act is expressed. "いれば" = If "いる" = now not 'いる' = now 'いない' The state not drunk is expressed. been like. "薬さえ飲めば" and "薬を飲みさえすれば" The act is expressed. now drink or drink in future. "薬を飲んでさえいれば" The state not drunk is expressed. "君さえ我慢してくれれば、全てが丸く収まる" "君が笑ってさえしてくれてれば、それでいい" "彼さえいれば、成功していたのに" "妻さえいなければ、彼女に声を掛けたのに"