Mars Eclipse is sticking to a non-existing version of Java. How can I change it?

For reasons related to the project I'm working on, I need to use Mars Eclipse and Java 1.7.
I've tried everything I can think of to set Eclipse to use this version of Java, but Eclipse will just point to a different JDK.
I tried overwriting the configuration via eclipse.ini. I've tried deleting all versions of Java I had and reinstalling only the JDK I wanted. I tried opening Eclipse from a terminal, while also trying to overwrite the settings from there.
As of now, I deleted every folder from all versions of Eclipse and Java and the relevant environment variables, and started from scratch (again). I have my JDK folder in C:, my %JAVA_HOME% is pointing towards C:/jdk1.7/bin and eclipse.ini has -vm¶C:\jdk1.7\bin (I already made sure I was using the right newline character and tried with both, single and double dashes before vm and \ and / for the route to the JDK bin folder). And when I try to open Eclipse, it crashes, as it's looking for C:\jre7\lib\amd64\jvm.cfg (jre7 doesn't exist). When I click OK to that window, I get another one saying

Java was started but returned exit code=1
C:\Windows\**system32**\javaw.exe
-jar
C:\Users\myuser\eclipse\plugins\org.eclipse.equinox.launcher_1.3.100.v2015
0511-1540.jar
-os win32
-ws win32
-arch x86_64
-showsplash
-launcher C:\Users\myuser\eclipse\eclipse.exe
-name Eclipse-mars
--launcher.library
C:\Users\myuser\eclipse\plugins\org.eclipse.equinox.launcher.win32.win32.x86_64_1.1.300v20150602-1417\eclipse_1611.dll
-startup
C:\Users\myuser\eclipse\plugins\org.eclipse.equinox.launcher_1.3.100.v2015
0511-1540.jar
--launcher.overrideVmargs
-exitdata 13a0_f0
-vm C:\Windows\**system32**\javaw.exe
-vmargs
-jar
C:\Users\myuser\eclipse\plugins\org.eclipse.equinox.launcher_1.3.100.v2015
0511-1540.jar

Note it references system32, a folder in which Java has never been installed or located in, and which has never been set by me in any Eclipse setting.
I've got a week trying to figure out how to fix this and I am the only one among my coworkers who has this problem. My OS is Windows 10.

There are two entirely different concepts of 'which JDK is eclipse using':

• Eclipse it itself an application written in java. Which JVM (java.exe) is it running on?
• When you develop java code in eclipse, eclipse needs a JVM for various purposes: To know that java.lang.String exists, for example. Or to run or debug this project. Eclipse can be configured with as many java installations as you want, and any given project can be told to use any VM you told eclipse about. The VM that runs eclipse does not have to be the same VM that is used for running / debugging java projects you're writing in eclipse, nor to serve as bootclasspath for these projects.

I'm having a hard time fathoming why you would have to run eclipse itself on a 1.7 VM. So don't; go to preferences, search for 'installed JREs', and add the 1.7 JRE. Then right click on a project, build path, and you can tell eclipse to use that JRE/JDK to run that project and to serve as bootclasspath for it.

The only real reason you'd have to run eclipse on 1.7 is if you are forced to use some outdated eclipse plugin that somehow doesn't run properly on 1.8+ - unlikely.

In that case, the -vm parameter is NOT the thing to use to point at the executable.

The proper argument is something like C:\Program Files\Java\1.7.025 - I'm not quite sure how the dir structure works out on windows installations, but it's supposed to be the directory within which you find 'java.exe', pretty much. And not the java.exe that points at the real java (i.e. the one in C:\Windows\System32 is not useful for this exercise). You also don't include the 'javaw.exe' in this.