non potuisset she would not have been able. This is the Pluperfect Subjunctive form of non potuerat she had not been able. Notice the force of the Subjunctive Mood in this circumstance. It makes the sentence hypothetical. Jessica, who has a strong grasp of her own strengths and limitations, didn’t even try to climb the pipe holding the suitcase. But she would not have been able to if she had.
tenens holding. This is the Present Participle. It is more commonly seen in sentences like this; Jessica sarcinam tenens per viam cucurrit Jessica ran through the street holding the suitcase. What we have here is subtly difference; she would not have been able to climb the pipe as long as she was holding the suitcase. If you examine the translation below, you will see that the Present Participle can be used in English in exactly the same way.
furtim secretly. This is not so much an observation, as an interesting list. Here are some words that share something of form and meaning with furtim secretly; fur thief, furax inclined to steal, furor, furari, furatus sum to steal, furtificus thievish, furtivus stolen, furtum a theft, furunculus a pilferer.
I was surprised to find that the great Cambridge Latin insult furcifer scoundrel did not belong on this list. furcifer is made up of two words, fer carry and furca. A furca is a pitch-fork, but also the name given to (and I quote Cassell’s here) an instrument of punishment, with two prongs, to which the arms were tied. So, a furcifer is someone who might be forced to wear the furca. Go figure.
But holding the suitcase with her hand she would not have been able to climb the pipe. She therefore secretly hid it in the dumpster.