There could be a historical basis to it, depends on what Flotilla you're in. In GWX the grids are random, but from a selection of about five which the game has to choose from for a given month and year, which are usually historically plausible for a specific Flotilla. I imagine LSH3 is similar. Whilst the main area of the U-boat war was in the North Atlantic they still operated far and wide: From the Arctic Ocean, Baltic, Caribbean, Mediterranean, Black Sea, South Atlantic, Indian Ocean and even off the coasts of Australia and New Zealand. The more far away places were for the larger Type IX boats. You'll need to be economical with your fuel during transit.
Here's an example of an actual BDU KTB from WW2 showing each U-boat's position for a given day for a selected period:
https://www.uboatarchive.net/BDU/BDUKTB30312A.htm
The lack of proper objectives has always a limitation of SH3. In reality the U-boats were constantly micro-managed by BDU on a daily basis. One thing I like to do is use a random number generator on a separate device (laptop in my case) to help choose what grids I need to patrol next. I won't go into all the details about it but it's a nice way of simulating receiving new orders by being a little creative.
Engaging targets en route to an objective was typical and expected, though there were some occasions when they had orders not to, but those were quite rare and usually limited to special assignments.
Someone from the LSH3 team would have to answer the last question.
