In Compose mode, put in some marker text which doesn't appear anywhere else in your post (I usually use XXX) at the place where you want to add the HTML.
Then switch to HTML mode, and use your browser's Find tool (it's Ctrl/f on Chrome) to locate that marker text.
Add your HTML.
Then switch back to Compose mode, and remove the marker text.
There is no way to prevent Blogger from doing this: the Post-editor does not support all aspects of HTML, and so some unsupported elements that you add will be removed (e.g., <p> tags), while others (e.g., tables) will be kept even though there is no way to create them in Compose mode.
I do not have a list of which HTML elements are and are not ...