Is it correct that keşke is always followed by -se/sa?

So that actually in Turkish you say ´I wish if...´ instead of ´I wish that...´?

Yes, keşke is always followed by the conditional suffix (se/sa) added to the verb.

My understanding from lessons and books is that ´keşke´ indicates a wish that is outside the speakers control, or whose fulfilment is known to be impossible.

The meaning given to ´keşke´ is often ´if only, would that, I wish´.

It tends to express regret that an action is not possible.

Whenever ´keşke´ is used, it is never in combination with future, aorist or present continuous tense suffixes on the verb stem.

Some examples (from a book)

Keşke onlar düğüne gelebilseler.

If only they could come to the wedding. (but it is not possible)

Keşke şu anda burada olsan.

If only you were here at this moment.

Keşke dünyada hiç savaş olmasaydı.

If only there were no more wars in the world.

In my opinion, for the second part of your question, the translation for some sentences using ´keşke´ would not be ´I wish if´ or ´I wish that´ but just ´I wish´ 

to express a "regret".. mmmm..  "keşke" is not the only option.. "ne olurdu" may sometimes work better..

Ne olurdu sanki doktora daha önce gitseydi. Doktor onu kurtarabilirdi.

If only he had seen the doctor earlier. He could have saved him.

Listen to this song..

https://www.youtube.com/watch?v=GDkkztdJOrA

Another song..

Ne olurdu benim olsan

if only you were mine

http://www.dailymotion.com/video/xewbmy