Hello, total newbie here - I'm very sorry if I'm even in the wrong forum

I'm trying to record lectures over powerpoint (which means: I put a powerpoint on the screen of my laptop, plug in a microphone, press record and start talking).

To record this, I'm using the free version of VSDC video editor. I then clip the beginning and ending in Windows 10's native video editing app (that used to be Movie Maker, but it's now something called 'Video-editor'). So, basically, this converts the original recording and then exports it to something usable.

Here's the problem
After exporting from Video-editor to for example an MP4-format, I've found the audio and video don't synch up. That's indicative of a recording in variable frame rate.

Looking back to the original recording I made with VSDC, I found that the root problem was in the recording itself: apparently, VSDC has as a standard setting that it records with a variable, not a constant frame rate.

Please help me out by...
So, I now have several questions:

Question 1. I must be incredibly stupid, but in the screen recording menu of VSDC I can't find the button to tell the software that I want a recording in constant frame rate. Here's what the screen looks like:

[Attachment 53426 - Click to enlarge]

When I select a framerate from the menu, it just gives me options for framerate, but nowhere can I force a choice for a constant frame rate.

What I'd like to know is this: if I choose a frame rate in this menu, can I assume it is a constant frame rate? i.e. if I select 29.97 as frame rate, will it be constant or will the software still secretly record it as a variable frame rate?

(and to add a bit: I am worried it is the latter. I distinctly remember choosing 29.97 before the recording that has the bad synch, which would imply it is a variable frame rate anyway, but I could be mistaken anyhow).

Question 2 Is there an easy way in which I can convert the original recording from a variable frame rate to a constant frame rate, in such a way the audio and video synch up perfectly?

The original recording that the free version of VSDC Video Editor makes is in .M4V-format

I already tried handbrake.fr based on some suggestions I saw on YouTube, but it didn't seem to do the trick.

Any other advice?
If you have any other advice, you're more than welcome to share

Thanks in advance for your help, much appreciated!