Interlocking Patches

Interlocking Patches to Crochet & Collect:

They're little pictures that you crochet. (Website)

Start with the Clover (It doesn't cost anything. There's a video).

Browse all 420+ (nice) Motifs in the Patch Index.

A month's subscription lets you download the whole catalogue to keep forever—that's crochet patterns to last until you're dead.

Not foolish at all, and the answer to all three questions is, yes! I do sometimes sketch things out that, on closer inspection, don't translate readily into crochet. But 500-or-so patterns in, I also have a good sense of what will "work." This can itself be a limitation because, to answer your third question, most lines really are crochetable, if you absolutely insist. (You can ignore any rule and circumvent any obstacle in crochet if you're willing to do something difficult, convoluted and/or ugly).

Usually an "illegal" line/stitch's only offence is that I don't have a way to notate it. One example would be "looong diagonals," which have always been possible but only recently made their published debut:

n/p stitches (triple crochet "regular" diagonals, 2 yarn overs) and nn/pp sts (double-triple "long diagonals," 3 yarn overs) had been the only diagonals permitted until this motif (S009-04 Fluted Bivalve), but only because I lacked an elegant way to specify how many yarn overs a longer stitch would require and where it would need to "go."

(It remains to be seen if the solution I came up with, above, makes sense to anyone but me, but I like it!)

If anyone wants to play around with the structure of interlocking crochet without having to actually touch yarn, I will always refer them to two Pals of Patches, Mizi and Ursa's Pattern Designer web app:

A031-10 Trail Aheads with Postage Stamp borders