DXF is a living, but ancient Vector Graphics format, coming from AutoCAD, I believe.
SVG (scalable vector graphics) is a much younger, more flexible format, used by everything from vector graphics apps to web browsers and comes with scripting within.
But I'm not an expert regarding either, so you may check with WIKI or the likes. I'm sure there are plenty of details to find!

What do you mean by "aligned with 3 vertex points inside"?
If you start a line or bezier, snapped to another tool, it will analyze where you snapped it on and provide a tangent as orientation vector. Holding shift allows you to lock to the tangent or any 90 degree (45 on some) step around it as you change your direction during dragging, while it also offers to lock to compass steps (canvas coordinates).
You may snap to any location on a line, bezier, circle or arc and get the corresponding tangent from that place. If you wish to consider this tangent to have 2 points and then consider your new line (or curve) to start also with 2 points, then you would have 4 points that compute their relationship.
YES, the notion exists to have merge. Not only for lines, but also for patternpiece polygons. It's not possible to merge Beziers, unless I introduce Splines, but I currently don't have that notion, because it is not necessary and only complicates things for no reason. But, yes, merging lines that are in a straight line or polygons that share an edge is definitely on the table!
