World View

Rectangles and Picture Frames share a common "last color used" default for their strokes. But you can change the stroke color of any rectangle or any picture frame to whatever color you want. They are not linked; you're just seeing a default or initial value when you draw them, just as if you drew multiple rectangles. If you don't like the default, change it as you draw the next object.

I just tried this - the picture frame outline changes its color based on what is behind it, making itself dark on a light background or light on a dark background, but there seems to be a "sweet spot" at 25% gray where the color of the frame outline matches the color of the background, making it seem invisible.
Try making your rectangle anything other than 25% gray and the picture frame should show up.
Only Serif can speak to whether or not this is something they will try to account for in a future update.

If the picture is in a frame and that frame is on a master page, make sure you are trying to delete the image itself and not the frame.  The frame would normally be locked by nature of the fact that it is on a master page, and one click on the frame with the move tool would select the frame containing the picture.

Or select it and use Send to Back... both methods do the same.

Move the rectangle below the other Master Page content, by dragging it in the Layers panel.

They show up for me, too.
Here's a Master Page with two empty picture frames, one selected and one not:
 

The attached file has history saved.
If you look in the first step of the History Panel you will see the Branching History icon.
Click this to toggle to the alternative history of this document.
You can switch back and forth and continue to work on both branches.
Indeed you can start other branches from any point in the history.
I'd imagine this could get pretty confusing with too many paths to choose from, and the file size may be come excessively large since you are recording every variation, but it does work in lieu of Snapshots.
Affinity Branching History.afpub

Publisher (in common with Photo and Designer 1.7) supports a branching history, that can be saved with the file, and can be used to some degree like snapshots.

Photoshop is a photo editing application and Publisher a DTP application - two different beasts. It's better to compare like against like rather than apples and Tow Trucks. Adobe InDesign not only doesn't have a snapshot option but doesn't have a History panel either and these two applications are normally compared against each other. I'd guess that a snapshot option for a DTP application is a lot more complicated to implement than in a photo editing app. It's also worth bearing in mind that Indesign currently costs £238 per year as an individual app compared to the likely £48.99 (or whatever the pricing for each Affinity app is at Publisher launch) for the release version of Publisher. I don't expect it to compare 100% with Indesign yet but already it has features that Indesign doesn't and runs so much faster.

Check your layers Studio Panel, do you have the Master Page applied twice? I ask because these sound like  problems I had where the Master was applied twice and again there was a case where I had made two text frames on the Master Page one on top of the other. The Layers' panel can be used to select items and delete them from there.

This is all expected behaviour.
You should have a very close look at into Publisher’s online help. Read the entry „Pages" and there „Master Pages“.
You can edit master page elements on the master. If you want to edit them on a layout page, open a layout page, go to the „Layers“ panel and right click onto the layer „Front Page“. You will find the entries „Edit Detached“, „Edit Linked“ and Edit Frame Content“.
The first entry allows you to edit the master element individually on this layout page, the second entry allows you to edit the element, but the modifications are reflected on the master itself, the third entry allow/disallows to edit the content of the element.
And: You can’t create a new text frame, because you have a large text frame on your master. Every time, you try to create a new text  on the layout page, the master frame is activated and you write into it (you allowed to "Edit frame content“).

I'd like to understand what you're trying to accomplish. Generally a Master Page should contain data that is static (except possibly for fields like a page number), and the document pages should have the actual text.
You seem to have two Master Pages with all of the actual content, and you've applied each of them to a document page to create a document page with the content.
In particular, I think your PAGE 2 Master should have had an empty Picture Frame and an empty Text Frame. Then on your Page 2 document page you would Place an image into the Picture Frame, and type text into the Text Frame.
What is also unclear to me is, when you say you can't change the size of "the" text frame, which text frame, on which page, do you mean?

Thank you Walt. I made two mistakes. 1. I had content on the Master pages. 2. I tried to delete a text frame when in text frame tool - works in Adobe, but here it's different, I had to move to the move tool to delete a text box. Solved, thank you.