Blender

Blender IS FUCKING LIT. But deep as hell.

Start blender, change the "screen block" (first dropdown to the right of the File-Render-Window menu in the far top-left corner) to Video Editing
Now you have three areas, what I'll call top-left "graph", top-right "video preview", middle "video sequence", bottom "video playback"; each has a viewbar at the bottom of the area; in the "graph" area, select the area type dropdown (far left) and change it to "properties"; note that strangely, the properties ares has its area type dropdown at the TOP of the area, doh

Set up Render

In the top-right preview window, you are viewing VSE output (looks like a filmstrip S shape)
Carefully change that to Properties (remembering that the fucking toolbar will POP from bottom to top of the window - you want to set it BACK to VSE when done configuring the render properties)
Now you can change "Render Settings"
- Dimensions > Resolution > (default is 1920x1080, you might want 1920x1920 if you have both vertical and horizontal footage); See Add a movie, below, to use Image Offset when importing to center each one accordingly
- Output > directory to (project)/output
- Output > ffmpeg video, RGB
- Encoding > Container > MPEG-4, Codec H.264
- Encoding > Audio Codec > AAC
- (used to have Presets > MPEG-4, Codec > H.264, Audio Codec > AAC (these codecs play on iphone + android + chrome + firefox!))

In the "video sequence" area, click Add > Movie to get a file browser and pick a movie file; I like to change the view to list and sort to by date by using the toolbar
Set the End frame in the video sequence to give enough width to fit the videos, eg 250 > 25000; use the scrollwheel to zoom out, and ctrl-scrollwheel to center the sequence
Set up smaller "proxy" videos for each video strip you import; right-click video strips to select them, in the properties to the right select [x] use proxy 25%, then click rebuild; hover over the clip, press N, select Proxy render size 25%; repeat for all strips

Annoyingly Blender does not use the camera metadata to rotate the video, and it's a pain, but easiest way after much experimentation is:

Video Sequence Editor (VSE) > Add > Movie > (select the movie as-is)
(rclick on video) > (make note of Original Dimension in far-right Edit Strip box)
VSE > Add > Effect Strip > Transform (while video is still selected)
Change the Rotation to 90; this will fuck the resolution
Change X and Y resolution to match the original video clip dimensions from above, swapping X & Y

A to unselect all, then B to "box select", then drag the mouse
On a strip...
- press K to cut it
- press X to delete it
- press G to move it left/right/across tracks
Click in the video sequence to set the current position; press M to leave a Marker there that can be returned to using the Marker menu