Ggtcf

At the time TeX was written, one page of a document would take several minutes to be processed, and syntax highlighting was not a thing, so it was a good thing to have some mechanism to detect if you forgot a }. A def, by default, doesn't allow a par token unless you explicitly say it's a longdef:

defmymacro#1#1

LaTeX, on the other hand, uses that by default, so if you use proper LaTeX commands (def shouldn't be used in LaTeX documents), newcommand makes a longdef by default. If you want a “short” def then you use newcommand*.

xparse returns the short argument default, but lets you define a long macro using the + argument modifier:

NewDocumentCommandmymacro m#1% def
NewDocumentCommandmymacro+m#1% longdef

Your second attempt is clever, and it could have worked except for two things.

First is that you are using gdefoldparpar and then gdefparoldpar. Once you expand par you get oldpar which, when expanded, yields par which, when expanded, yields oldpar which, when expanded, yields par which, when expanded, yields oldpar... Running forever :/

You need to use let (or globallet to have global effect) in this case: letoldparpar. This creates an exact copy of par named oldpar which does not depend on what is par.

Second, the runaway argument checking is implemented in a lower level, independent of the definition of par, so this would fail with the same error:

letparrelax
defmymacro#1#1
mymacrofoo

bar

because when TeX sees two endlinechar tokens (which is a space by default) TeX inserts an implicit par token, which raises the Runaway argument error. Knowing that, then:

newcountoldELchar
oldELchar=endlinechar
defmymacroendlinechar=-1relaxmymacroi
defmymacroi#1#1endlinechar=oldELchar
mymacrofoo

bar

won't raise an error, but a new line won't be a space anymore.

The argument of a macro defined with def does not allow par tokens. Neither it allows blank lines, because they're transformed to par during the phase in which TeX processes text input into tokens. Note that redefining par is useless in this respect, because it's precisely the token par that's disallowed, independently of its meaning.

Solution: make your macro long.

longdefmymacro#1#1

Better solution:

newcommandmymacro[1]#1

because newcommand uses longdef internally. The variant newcommand* instead uses def without the prefix.