You are correct. AutoCAD expects metric units (though if you give it meters when it is expecting mm or vice versa you can get strangely large of strangely small measurements - see here). Giving it a dataset that is in a GCS (WGS84 in your case) just compounds the confusion as you rightly surmise (decimal degrees do not readily convert to metric units as it depends where on the globe you are).
If you are still getting really small measurements check two things: Firstly, have you exported the data with the correct projection? Don't change its CRS in QGIS on opening the layer but specify the CRS you want when you Save As
. Secondly check (using the link above for more information) that AutoCAD is set correctly to meters and not mm.
Is there a tool or equivalent algorithm for the logical fourth one: "Merge Parts"?
No, there isn't.
The adjacent parts of a multipolygon are considered a topological error and must be linked with the Fix geometries tool.
Is it even possible to select individual parts?
No, it isn't.
You can select features, not individual parts.
Is there anything else I'm missing?
I don't know. But you can refer to parts of collections with the geometry_n()
function in expressions.
For example, you can create a new feature (in a separate layer) with the Geometry by expression tool, that contains the combination of two parts (or more if the function is nested) of a feature or of several, of any layer. If the parts are adjacent, they will form a single part in the new feature.
To create a feature formed by parts 1 and 2, of the feature_ID 0, of the 'multipolygons' layer:
combine(
geometry_n( geometry( get_feature_by_id( 'multipolygons', 0)), 1),
geometry_n( geometry( get_feature_by_id( 'multipolygons', 0)), 2)
)
Note than the Geometry by expression tool copies the table from the input layer. If the input layer have more than one features, more than one features will be created, but only one will have a valid geometry with that expression.
UPDATE:
About the WKT of the Multipolygon provided in the comments (https://pastebin.com/1ibYDzR5), I can't seem a line shared by both parts.
When loading the feature in QGIS, it can be seen (with an appropriate zoom level), that the parts are not adjacent.
Upper:
Lower:
Be sure to snapping when digitizing and editing (here is the manual). Otherwise, you can not merge those parts as they are in any way.
Best Answer
As gene mentioned, there is a Spline plugin to digitize curves. Control is limited but it might be a start. Check the plugin settings and adjust the tolerance value to fit your layer CRS and resolution, especially if the CRS is EPSG:4326).