A: In theory yes, but it has a high chance of failure. If the hack uses UberASM code of its own, patching this will remove it and break things. You can get a higher chance of success patching the original Retry patch, but it has been proven to break certain stuff (like changing how sprites spawn on level reload, since the hack wasn't designed with Retry). In general I wouldn't recommend editing someone else's hack: if you really want to save some time, just use save states.
A: No, doing so will result in an error when running UberASM Tool. First, you need to remove the original patch: either try to use the Unpatcher tool (at your own risk) or port your hack over to a new ROM.
A: Yes. If you want to disable infinite lives, you can use the !infinite_lives global setting (see Global Settings - Quality of Life Settings). If you want to keep infinite lives just for specific levels, you can do so using the %no_lose_lives local setting (see Local Settings - %no_lose_lives).
A: Yes. Look at the Local Settings - %checkpoint page.
A: Yes. Look at the Local Settings - %retry page.
A: You can do so with the !death_sfx option in settings_global.asm (see Global Settings - SFX for details). You can also find a custom death sfx file that replicates the vanilla death music intro under resources/amk/sfx (see AddmusicK Resources for details).
A: Yes. Additionally to the effect table, there's the !retry_ram_prompt_override address that, when set, will replace both the table and the default option (see retry_config/ram.asm and RAM Map).
For example, you could use it to let the player choose the prompt type as they want (you still have to implement that, though, as this only handles setting the prompt type).
A: Yes. Look at !no_prompt_bg in retry_config/settings_global.asm. You can also remove it only under certain circumstances by setting !retry_ram_disable_prompt_bg to 1 (for more info, see retry_config/ram.asm and RAM Map). For example, you can do it in a Level ASM init to make a level not have the option.
A: Yes. Look at !no_exit_option in retry_config/settings_global.asm. You can also remove it only under certain circumstances by setting !retry_ram_disable_prompt_exit to 1 (for more info, see retry_config/ram.asm and RAM Map). For example, you can do it in a Level ASM init to make a level not have the option.
A: Yes. Depending on what type of Retry prompt you're using, you can use the !prompt_box_text_x_pos and !prompt_box_text_y_pos options or the !prompt_bar_text_x_pos and !prompt_bar_text_y_pos options in retry_config/settings_global.asm. For example, using X pos $08 and Y pos $CF will place it on the lower left corner of the screen.
Additionally, you can also change it at runtime by writing to the !retry_ram_prompt_x_pos and !retry_ram_prompt_y_pos addresses (see RAM Map for more info).
If you're using the prompt box and the black bg is enabled (!no_prompt_bg = 0), you're a bit more restricted in how much you can change the position. For once, the Y position shouldn't be changed, while the X should be kept in the confines of the black box (also accounting for how long the prompt text is).
If you're using the prompt bar and the black bg is enabled (!no_prompt_bg = 0), you can more freely change the position, but also make sure that you set !prompt_bar_position and !prompt_bar_size so that the text will be inside the bar (basically, !prompt_bar_text_y_pos should be >= !prompt_bar_position and < !prompt_bar_position + !prompt_bar_size - 8).
A: Usually you can just try messing around with the values until they look right, but here's a short explanation.
All the position values refer to the position in pixels on the screen, where the up left corner is position X=0 Y=0. Higher values of X mean going more to the right, and higher values of Y means going downwards. This means that with the standard NTSC SMW, the max position is X=255 Y=239 (for the thing at that position to be still visible, although just for 1 pixel in the bottom right corner).
Note that the positions always refer to the first tile of a particular element (like in the text position, it refers to the cursor) and it will place the top left corner of that tile at the position specified.
A: Sure, you can freely edit all of the graphics used by Retry, which can be found under retry_config/gfx. For more details on the files found there, check out Retry Resources - Graphics.