Try isolating the problem, see if what you passed is getting there.
They should match.

from the launcner:
printf("n_argc=%i, n_argc=%p, &rshell=%X\n",n_argc, n_argv, &rshell);
((void (*)(int, char **, rshellAPI*))(docptr + sizeof(PRGMSIG)))(n_argc, n_argv, &rshell);


from the launcned:
int main(int argc, char** argv, rshellAPI* rshell){
 printf(argc=%i, argv=%p, rshell=%p\n",argc, argv, rshell);
.
.
.
}

Alignment errors: The adressess in argv, rshell should end in: 0,4,8,C


Declaring with "static" might solve the alignment problem:
static rshellAPI rshell =
{
   &c_draw,
   &c_clear,
   &c_writec,
   &c_write,
   &c_swrite,
   &cn_readc,
   &c_readc,
   &c_read
};

Yes, that works for void functions.
The advantage with lambda is that it allows you to pass arguments
should Spyro543 decide later to do so on fopen:

browsebtn = Button(root, text="Browse", fg="black", command=(lambda: fopen(arg1,arg2,...)))

Up till now, to print text on the Nspire screen required you to bring
your own Font table into your code. This is not always convenient.
The text is small, but a simple OS call: disp_str(char*, int *x, int y) prints a string where you want it
without bringing in extra screen routines and a Font table.
Maybe this can be used in the Ndless console program ?

Here is a simple example I found for Ndless 1.7 and  2.0.1.60

 

bsl, could we have some photos of your dock connector which looks very interesting to me?

I got a problem with my camera driver,  but I think my description is easy to follow.....
I will try to get photos later...

I was going to build the circuit , buts its easier and cheaper
to order the circuit  http://www.ioffer.com/i/USB-to-RS232-Module-Based-TTL-PC-FTDI-FT232BM-BL-chip--67532016
Let me order it and get it up and running and then i will get back with you.

Its been a year since I touched my HP50g , but you may be mixing ARM with Saturn
assembly language.
You might have to start with !ARM

As a side note: The Saturn processor has a 500K memory limit. Why Hp didnt put more memory on with the 49+,
when they emulated the Saturn on that calc is beyond me. You can switch to ARM mode with more memory available

Here are more unconfirmed calls.
The sprintf routine is not a syscall, so its OS dependent.
I will try this myself in a couple weeks, it looks too useful even if its not sprintf.(vsprintf ?)
MMU_FlushCache?               0011
Disp_Manage?                  0276
Disp_Save?                    0278
Disp_GetPtr?                  027A
Num_UintToBCD                 0577
Num_BCDToUint                 0578
strlen?                       060D
strcpy?                       060E
strcat?                       060F
is_comma                      07BD
Bkey_GetkeyTableInfoJumpFunc  0EAE
Expressions_OpcodeToStr?      1291
LocalizeStringID              12FD or 12FC
MCS_DirtypeToItemtype?        1549
MCS_ItemtypeToDirtype?        154A
MCS_OpenAlphaMemItem??        1553
NextOPcodePRGM                1749
MCS_Flush                     1845
Send38k                       198A
Receive38k                    198B
OpenCloseComPort38k           198C
Battery_IsLow?                1e72
GetIntPtrContent?             1E7D
GetdatatablePtr?              1E80
SetAppName?                   1E9E

sprintf??? -- non-syscall  --> (OS1.01:0x80309604, OS1.02:0x803093A0), use function pointer   

Here is another, not in the demo, but necessary to complete communication between Basic and C:
#define Alpha_SetData(VarName,Src) (char)(*iSysCallFuncPtr)( VarName,(int)Src, 0, 0, 0x0035)

Some other untested syscalls, I have found:
Setup_GetEntry 0x0489
Setup_SetEntry 0x048B
Alpha_ClearAllAndAns 0x0A8C