I grant you that the user interface for merging Places vs. Merging Place Details can be a little confusing. I have complained about it but it has never been improved.
Here's the tricky part. When you are in the Place List and when you highlight a Place, it shows you any and all Place Details associated with that Place. It looks like you can therefore do things like highlight some Place Details and merge or delete Place Details from where you are. But you can't. Before you can do anything with the Place Details, you first have to click on the Place Details tab. Having done so, you will see a sub-window called Place Details. Only when you get this sub-window can you manipulate the Place Details.
So be really, really sure that you click the Place Details tab and be really, really sure you get a sub-window that says Place Details - not Places. If you get to that point, it should work just fine to Merge or Delete any of the Place Details.
P.S. It's not clear how best to change the Place and Place Details user interface to make it less confusing. The trouble is that you can edit a Place Details by double clicking it and without first clicking on the Place Details tab. But you can't delete or merge Place Details unless you first click on the Place Details tab. It may seem like a difference without a distinction, but I think that's the root cause of the problem. And if you removed the ability to edit a Place Details without first clicking the on the Place Details tab, I'm sure that users would complain about the extra click to edit Place Details (and I'm a prime critic of RM's clickiness). But I think that edit, delete, and merge of Place Details all need to be in the Place Details window instead of edit of Place Details being in the Place window.
RM stores its data in an SQLite database, and the data we are talking about is stored in TEXT or BLOB fields. The default limit for these fields is a billion characters. I don't know if the RM database is using this default or not, but in any case I'm sure it's huge. As Tom said, it's so huge that you should decide on your own practical limit because your practical limit will be far less than the technical limit.
.... but I'm not sure how you would embed a <carriage return> or <new line> command.
RM's sentence templates can contain a new line. The new line is stored in the RM database as two characters - a carriage return followed by a line feed - CTL-M followed by CTL-J (or 0X0d0a, if you prefer). But the way you enter such a two character sequence is very simple. Just depress the Enter key on your PC's keyboard a single time. The carriage return and line feed sequence will be invisible on your RM screen, but the effect will be visible because you can see that the template has advanced to the next line.
I'll have to think about what my true objective is for this person's record.
I think most everybody has to think pretty seriously about their true objective in recording their family history. I like to think my true objective is the production of reports, either printed or via Web pages. That's more important to me than is getting WebHints or anything like that. I find that I make data entry or research decisions on a regular basis that best meet my reporting needs but that might be sub-optimal for other purposes. Everybody's needs will be different, and there is no one size fits all solution for a lot of these questions.