External Markdown Editor

Is it possible to use a Markdown editor different from MacDown?

I have got Markdown+ but it crashes if I
a) set all md files to be opened by it [using Finder] and
b) try to open a file that is stored in a DT3 database by clicking the correct icon in DT3. [Arrow pointing up and right inside a square]

You can use Typora, for example. The issue of external MD editors is regularly discussed here, as a search in the forum will tell you. Browsing through the search results should give you some options.

I use Typora with no issue.

There is no requirement to use MacDown. It’s what I cut my teeth on when teaching myself Markdown as it’s a free and capable editor. That being said, you can use what suits you. Just be wary of any proprietary Markdown features an app may implement. We support MultiMarkdown 6.