Here is my solution for the first point : the insertion step. I wrote it in bioperl. It's not super clean but it works :

#!/usr/bin/perl

use warnings;
use strict;
use Bio::SeqIO;
use Bio::Seq;

my $file = $ARGV[0]; # input fasta file (genome file)
my $out = $ARGV[1]; # output fasta file

my $chr="test"; #insertion chromosome
my $pos=10; # position of the insertion
my $seqI = "AAAA"; #sequence of the insertion

my $seq_in  = Bio::SeqIO->new( -format => 'fasta',-file => $file);
my $seq_out = Bio::SeqIO->new( -format => 'fasta',-file => ">".$out);
while( my $seq = $seq_in->next_seq() ) {    
    if($seq->primary_id eq $chr){
        my $length = length($seq->seq);    
        my $upstream=substr($seq->seq, 0, $pos);
        my $downstream=substr($seq->seq, $pos,$length);        
        my $seq_obj = Bio::Seq->new(-seq => $upstream.$seqI.$downstream,-display_id => $seq->primary_id,-alphabet => "dna" );
            $seq_out->write_seq($seq_obj);
    }
    else{
        $seq_out->write_seq($seq);
    }
}

For the purpose of adjusting the GTF file it should be sufficient to

• shift all genomic coordinates (start, end) with start,end >= insert_start by the insert size (that means add the insert size)
• leave all other coordinates untouched
• if insert length is not a multiple of 3, frame information needs to be adjusted for all regions that have frame information and a start position >= insert_start, if frame > 0. For regions with frame < 0, adjust frame if end >= insert start. This can be calculated as new_frame := (old_frame + (insert_length %% 3)) %% 3