"A dime a dozen" meaning cheap or common is a poor choice of idioms here considering what she really went through.
Remember the case of Jesus and his followers? Consider Peter. First he speaks well and Jesus calls him blessed:
"He saith unto them, But whom say ye that I am?
And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven." Matt 16:15-17
Then a few verses later Jesus rebukes him calling him satan:
"From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee.
But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men." Matt 16:20-23
Peter simply wanted Jesus to live and continue being Master in the here and now. He did not understand God's purpose with the death of Jesus... not yet he didn't.
We may not understand God's purpose for allowing Dr Helen Roseveare to suffer as she did. Was there a purpose for it? Well if we first consider that she loved God through it all and then read this verse we may understand that there was:
"And we know that all things work together for good to them that love God, to them who are the called according to his purpose." Rom 8:28
Does that verse apply to her? I would certainly say, yes. But still we do not know all of the answers. She had no machine gun, but she was undoubtedly in God's will for her no matter how difficult and painful it was. A machine gun might have saved her body from a painful experience, but what would she have lost that she gained with God? God knew and maybe she did before her time here was finished. Very likely she does now.
Could it have been done another less painful way"?
Could God have accomplished His purpose for us with Jesus in an easier less painful way? Remember Jesus' prayer? He did not want to suffer and die in such a painful manner, but he prayed for His Father's will instead his own:
"Then saith he unto them, My soul is exceeding sorrowful, even unto death: tarry ye here, and watch with me.
And he went a little further, and fell on his face, and prayed, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt." Matt 26:38-39
So then your question: "What does it all mean?"