Is it permissible to combine the Dhuhr and Asr prayers by delaying Dhuhr due to difficulty reaching the mosque? And is leaving the city considered traveling in this case? And is it permissible to delay the Jumu'ah prayer and perform it as Dhuhr with Asr?

What your friend mentioned is incorrect. The distance you specified is not considered travel by the majority of scholars. The distance for which it is permissible to shorten and combine prayers is four burd (approximately 83 kilometers), or what is customarily considered travel. The distance you mentioned is not considered travel customarily, and therefore, it is not permissible for you to shorten or combine prayers.

As for Jumu'ah prayer, if your workplace is within the city limits, it is obligatory for you. If it is outside, and the distance between you and the mosque is a farsakh or less (slightly more than 5 kilometers), Jumu'ah becomes obligatory for you. If it is more than that, Jumu'ah is not obligatory for you according to the Hanbalis, and you should pray Dhuhr.

As for congregational prayer, you have an excuse for not attending it due to the distance. However, if it is possible for you to pray in congregation with your colleagues, it is obligatory for you to do so; otherwise, pray individually within the prescribed time.