A few thoughts and observations in arbitrary order. Some parts tackle your questions directly, others try to provide a broader perspective of where to go and how to arrive there.
Fretboard limitations wrt chords
Compared to a piano a guitar is quite limited. Think of e.g. the D-major chord, e.g. from here https://chord.rocks/guitar/chords/d-major , trying all "Variations" buttons. What's easy on the piano keyboard can turn into a nightmare on the fretboard.
So a way to come closer to sound of chords on guitar can be to translate these (a selection of relevant chords to your band) limitations to your keyboard, as boundary conditions for you: where you could do more on piano, a guitarist will soon face a harder time.
Like on piano/keyboard there's also fingering on the guitar, i.e. easy and impossible routes to change finger postions over time.
In a nutshell: know those instruments involved.
Fretboard and melodies/leads
There are similar limitations, but are less severe in my view. The solist simply has more options to "play the same note", while it will sound a bit different from fret to fret, string to string. See e.g. here: https://chord.rocks/guitar/scales/d-major .
Guitar sound ...
... is certainly something you hardly can replicate on piano. Though keyboards may provide some or many guitar sounds, like samples, sound variations from a real guitar (A or E) will vary much wider, and with lots of pedals s/he will outperform any keyboard mimick easily.
Solo vs. band approach
As I see it, in a functional band it should be sufficient, if one member describes a musical idea, the right person picks it up and after a few joined attempts it shapes out, how to do it.
Think of passing the ball to the team in a ball-game, like soccer, football etc.
This requires a certain level of skill from everyone, of course. However, work with the current level, as that's what you have available.
This is certainly the drummers task, calling etc. S/He should be literate in a few basic styles of the genres you play, just to synch-in the band, and to provide a rhythmic base to play from.
And with good listening skills from the drummer, you can avoid many musical cliffs, which appear every now and than ...
But also each genre singled out a set of must-know patterns for each instrument. E.g. how does reggae look like on key, string etc.?
Less is more - more or less
Related, there are two extremes of playing note ensembles:
- every instrument throws as many notes as possible (with overlaps in time)
- the complexity of the whole song is fragmented accross instruments.
The first approach you can find e.g. in orchestras: just look at a leadsheet or listen to an orchestra.
But you also find a transition to the second approach: e.g. wind-instruments can't play chords, so the chords notes are split and assigned to different players, like horn1, horn2 and horn3.
The second approach should be familiar to your drummer as so called linear-drumming. But you can also hear it in many bands from e.g. South America, Africa etc. as a kind of "linear-instrumenting".
To illustrate with linear-drumming:
- it's one pad at a time (pad=snare, toms, cymbals etc.)
- rather than "all at once" (whatever two arms+hands, 10 fingers and 2 legs can provide simultaneously)
- which results in a clear, crisp rhythm with a unique sound
- however, at times you need to "hit them all" for muscial reasons (emphasis etc.)
The rule of "don't stand in my way musically" can be seen here, too. I.e. it doesn't make sense to play the same frequency range as a drummer on the kick drum as the bassist does: one of them will not be heard. There's a time for each instrument to switch from regular voicing (git: strum-strum-strum, drum: rock-rock-rock) to stand out in a little solo or break (the others are quite, i.e. make room to be heard). Think of talking rules in a group.
Composing might also benefit from doing less, but clear. Viewed from the audience, complex songs are often hard to follow. Dance-ability can be a simple way to "measure" it, where "dancing" more means body movements, and less standardized moves.
You can fool the audiences expectations on the genre only a few times. E.g. providing a waltz in a reggae song, or "phantastic odd meters", can work, if you use it as you would use spice for a meal. It can sound great, or become a desaster ... It depends.
Besides all those rhythmes, notes, chords and so on to me interesting songs tell a story with musical, not neccessary lyrical means.
E.g. from listening to boring songs (the same again again ... wake me up, when it's done) and to enticing ones. The latter often make the drama "told" audible. Try identifying some generic patterns you can work with in your specific band environment.
Hope this helps a little.