MachStdMill (MSM) requires mach 3.43.22 as minimum revision level. This is stated on download page for the software, in the MSM readme file and again during the installation process.
The source for current mach revs is not the mach FTP site. The FTP site is where Artsoft keeps older (i.e. obsolete) mach versions. You need to go to the mach DL page to get the required version of mach.
There are two version of mach available from the machsupport site DL as of about 5 minutes ago:
The mach DL page URL is ArtSoft USA - Software Downloads
If you attempt to start MSM with a mach version earlier than 3.43.22 one of two things will happen:
a) if the mach version is old enough to not have the interface that lets a mach script get the mach version, you will see a message "Scripter error" when loading MSM. The message can't be improved as Mach emits this message before the MSM load script can be run to find out what the running mach version is.
b) if you run a mach version new enough to have the required interface, MSM will know the mach version running, see that it is too old and refuse to run - but it will tell you why first.
So step one to run MSM should be for you to DL and install the required 3.43.22 mach version.
You said you ordered and installed a MSM license. Please contact me by either direct email or PM with the license ID you purchased and I will verify the status of the license.
If you are running the 10x7 resolution screens the Override sliders are on the fly out page, if you are running 12x9 they are also on the run page. This is covered in the MSM user manual in several places including chapter 3 which covers the basic Mach controls.
The actual Feedrate will vary as a program is run - that's normal. In Gcode you set a target feedrate. Mach tries to keep that target but the actual feedrate at any instant will vary as the cutter changes direction, accelerates, decelerates etc.
I suggest that you install the mach version that is needed for MSM. Also be sure to read the README file so that you correctly set up a profile for use with MSM.
Several of the other things you listed sound to me like PC configuration issues (Mouse movement changes RPM, PC freezes).
I would suggest that if you are setting up a new control PC, that you want to narrow down the possible causes of problems. A good approach for that would be
a) install mach 3.43.22
b) run just the standard mach 1024 screen set
c) become familiar how mach operates (re feedrate variations etc)
d) After you have a stable stock mach configuration, then you can start adding in other variables (such as running MSM).
Once you are running compatible software versions, if you still believe you are experiencing an MSM problem, please drop over to the MSM user forum at Calypso Ventures Software User Groups - Home and I'll work with you there to take care of any issues.
(as part of being a good online neighbor, I prefer not to do lengthy MSM support threads on other people's forums).