1). I have tried using Bambu Studio in the “Keep only one Bambu Studio instance”. It works, but the UI always jumps to full screen mode. Is there a way to alter it so that it opens the previous window size and position?
2). When I perform “Slice Plate” I always get the Color Scheme drop-down opening at the top right, Is there a way to set things so that the drop-down doesn’t open every time, unless I click on it?
Let’s assume that you are referring to the Windows version since you didn’t mention it. Windows placement is a function of the operating system and not typically done by the application program. Some programs, very few in fact, do allow for resize on opening as let’s say to optimize for the application, but I have not seen a program that does this for just opening and closing. If one thinks about it, why would a developer waste time writing code for something that the operating system is expected to handle?
The quick answer is; no, this is not a feature currently but feel free to post it in the suggestions so it can be officially ignored by Bambu. 
All kidding aside, your request falls into what developers call “quality of life” enhancements—those small, thoughtful touches that make software a bit easier to use. Honestly, you’ll probably get more traction with Orca Slicer on this. Unlike Bambu, the folks at Orca seem dedicated to making the slicer more user-friendly, enhancing Bambu’s original code with features that actually benefit hobbyists. That’s because they are fellow hobbyists.
Bambu, on the other hand, seems laser-focused on pushing printer sales, complete with the unskippable Makerworld landing page on Bambu Studio—something Orca Slicer thankfully lets you bypass. It’s pretty clear where their priorities lie.
@Olias thanks for the reply. As to “Keep only one Bumbu Studio instance” “checked” and “unchecked”. When I’m designing something in Fusion 360 it tends to be an iterative experience. When this option is “unchecked”, I end up with unwanted Orcaslicer windows. OK, so I can just delete them, but at least they open in reasonably sized windows. When the option is “checked”, a new full screen window appears, and I am using a 57" ultrawide screen. OK, again a couple of clicks restores some sanity.
But I don’t understand when starting Orcaslicer normally, it opens a repeat of the previously used windows size and location, but using the “checked” option it opens full screen. This is surely a programming error?
Anyway, a few clicks corrects things, it’s just irritating.
But one good benefit that I have discovered when using the “checked” option. Whilst using Fusion 360 with several components in different windows. I can print chosen windows/items to OrcaSlicer, and they all land in the middle of the same plate. A quick click on “arrange all objects” and I can print them. So there is an unexpected benefit to using the checked mode. 
I will try asking the OrcaSlicer community, but it’s a bit of a none issue.
I better understand your quest. I share that quest and I seek such options with each iteration of windows going back to Windows 3.1. So I feel your pain😉
The term “error” has become a bit hard to define term. I like to call an error something that the designer intended but went awry. What your seeing is actually caused by when the program calls for the spawning of a new window or child window. Depending on how that procedure is called, it is very difficult for a developer to discipline the OS into doing it the way they want without writing a whole like of “hack code” as I call it to obey my wishes. I’m not making excuses but merely an explanation. You can tell that I’ve tried this in the past when I used to code but that was pre-windows 8.
There was once a utility called “Fix Windows Annoyances” that effectively addressed various user concerns. The name of the program alone speaks volumes of Microsoft’s GUI deficiencies. Unfortunately, Microsoft has become increasingly hostile toward its user base. They have actively developed code to block registry hacks that improve the user experience in Windows 11, effectively disregarding the power user community. Microsoft’s corporate attitude seems to align with Steve Jobs’ philosophy that “users are too stupid to know what they need, and we know better than them.” This approach began with Windows 8, which faced such severe backlash that Microsoft had to reinstate features like the Start menu. Since then, they have adopted a “boil the frog” strategy, subtly introducing these changes in each update and making users feel as though they are at fault.
However, there are a couple of tools that come to mind that while they won’t fix the launch position per se, they will quickly allow one to manage the desktop space using hotkeys. I’ll go through the one’s I know of and the ones I use or have used. Some of these will behave differently based on the video card you have.
You mention you have a 57" monitor. So do I and I must assume it is the Samsung G9. I run an NVIDIA RTX 3080 TI graphics card and NVIDIA has it’s own sins we must content with AMD Radeon is not much better but NVIDIA I found to be more stable. What I list here may differ if you don’t have my setup.
Windows 11 Snap Layouts – You likely already know about this but it should be mentioned here for completion.
Samsung Easy Setting Box – This is only for Samsung Monitors but may work on others. I have it loaded and I’m not a huge fan of this but it does allow for placement of windows in a region by dragging it near the title bar. It tries to emulate the Windows 11 Snap Layouts feature and does a poor job in my view but for Windows 10, its an option. LG has a utility for their monitors who’s name I cannot remember that supposedly does a better job.
Stardock Fences – I’m not huge fan of this paid for utility but it is specifically designed to allow one to micromanage one’s desktop windows and folder appearances. This may have some of the features you’re looking for. There’s a 30 day free trial. I visit it again from time to time and in my view its more of a solution looking for a problem to solve but you may find it useful. The titles are hotlinks.
Fancy Zones part of Windows PowerToys – Anyone serious about seizing control of their desktop like a Power User already knows about PowerToys and each iteration of Windows has one. This collection of utilities is a must have. The Fancy Zones utility is a CTRL-click and drag utility that allows one to drag and snap open windows into user-defined areas. For a 57" monitor, it’s a must have. It’s like Snap Layouts on steroids.
Sizer 4.0 by Brian Apps – This is a free utility that some of my PowerUser friends swear by. I can’t say that I found a use for it that wasn’t already done by Fancy Zones except one thing. It allows for hotkeys. What I don’t know is if it allows for window positioning too but it should be included in this list.
There are some older utilities designed for Windows 7 such as Winsize2 which are still around and allow for hotkey placement of window sizes. They may still work but I no longer use them. They are clunky in that their appearance is using the old style Windows 2000 GUI libraries so it will look more like a product from the nineties. A web search of windows display management tools will probably find others but most of those haven’t been updated in over 15 years.