[SalesForce] Notify Lightning Component when ApexBatch finishes

In the start method, a database cursor is created. You can think of a database cursor as a temporary table that provides consistent results across the life of the cursor. While it is true that this implies staleness, it also implies a proper snapshot-- all of the data as it was at a specific point in time. Moving that query to execute will cause you to have possibly inconsistent data.

For example, let's say that John Doe worked at Contoso, and now works for Acme. Let's also suppose that the query was alphabetical on account name in ascending order. We'll also say, for sake of argument, this change was made to John Doe during the snapshot process (between the start time and the time it got to his company). With the entire query in the start method, the snapshot would show John Doe only once, working at Contoso, which is out of date, but would be captured on the next snapshot. Conversely, live queries would miss John Doe if Acme was queried before he was updated and Contoso was queried afterwards. Conversely, if he was moving from Acme to Contoso, and was moved after Acme and before Contoso, he would appear in the snapshot twice.

That said, moving the query to execute would speed up the initial query, because it takes less time to execute many small queries than it does to create a larger cursor. Just be aware that it will be more likely to miss data or duplicate data in some scenarios. If records can't move, then this is less of a concern, although data will still be skewed.

Your main problem is that you should be returning the job Id to make sure you have the correct batch (more than one batch could be running). Second, imperative apex records return a different structure than that required for getFieldValue. Simply access the fields directly, or use a wired getRecord method.