I am currently writing a novel in journal format. It’s a teenager’s realistic journal so entries might range from 2-3 sentences to 10 paragraphs a piece.
Basically, the writing is separated into chapters just like any other book, but within those chapters, there might be maybe 5 to 10 journal entries. And sense the entries range in lengths it wouldn’t make sense to separate certain entries like scenes or chapters (because multiple journal entries might be part of a single scene).
In a perfect world, I would just be able to delete the indent on a single line and bold and underline the dates and times of the single journal entries, but I am not able to delete the indent on any first line (without having to change my default settings). Because of this, my manuscript looks really confusing in the editor.
Is there any way to delete a first line indent so that I can make headings within my scenes?
If all you want to do is remove the first line indent for a specific paragraph, this is easily done via the ruler in the editor. Format > Show Ruler to make it visible, then just drag the first-line indent marker (the triangle pointing down) all the way left. Ruler settings work per-paragraph and should be familiar from most other word processors; the settings will change for the paragraph where the insertion point is or for any selected paragraphs, and new paragraphs will pick up the formatting of the previous paragraph. You can also use the keyboard shortcuts Ctrl-T and Ctrl-Shift-T to increase and reduce the first-line indent respectively.
With this set up of putting the formatted subheaders within the text document, you will want to compile without overriding the formatting; otherwise, all your paragraphs will be standardized to either a first-line indent or none. That means that you’ll want to do your other text formatting within the editor as well–line spacing, font, size–and then uncheck the “Override text and notes formatting” option in the Formatting pane of File > Compile.
A different way to handle this would be to keep each of the journal entries as its own document in the binder. You suggested that the varied length of these makes them not worth having as separate items, but really there’s no reason within the program for that to be the case, and in fact you could get a lot of benefit by having the entries separate. With Scrivenings mode you can still view multiple journal entries together in the editor, and by keeping them separate you’ll also be able to more easily rearrange, remove, or insert entries; to make collections containing only the entries that meet relevant criteria (without also including the other entries in each scene); to toggle individual entries for compile (for instance if you’re trying out variations of the scene); and to give each entry its own metadata if desired. You’d also be able to use the outliner and corkboard to increased advantage by being able to see what and how many entries a scene is made up of, if you structure your binder such that journal entries are subdocuments of scenes which are subdocuments of chapters. And of course you can always collapse the scene containers to reduce clutter when you don’t need to see all their subdocuments.
The particular advantage of this method as far as your formatting goes is that you’d be able to use the journal entry’s document title as its subheader, allowing you to format it separately from the text–so in the compile settings, you would check the boxes to include both text and title elements, then format the title to be bold, underlined, and without a first-line indent. With the binder hierarchy suggested, you would also be able to format scene titles and chapter titles separately and have separate prefixes for them (for instance if you’re using prefixes to give you the “Chapter One”, “Chapter Two”, etc.).
The only caveat here is that presently there is not a way to have titles and text on the same line, so if that is critical to your formatting, this won’t work for you. And of course you should feel free to use whatever method best suits you. 
I just wanted to throw out the other option, since it’s a bit more of a Scrivener philosophy and less something that might naturally occur when you’ve been used to writing in other programs like Word.