hook_form_alter for the checkout_form

I needed to make some customization on the checkout_form in ec 5x-3.6, i tried building a simple module with a hook_form_alter as usual...
I noticed how difficult would have been to place a new form element inside the address fieldset, both the shipping and the billing one.
But i tried...
Normally you would use some simple array_slice and array_search to find the key where you wanted to place your new form element but that's impossibile with checkout_form this way, because you can only decide to place it before or after the fieldsets.
So i managed to go deeply inside the form to find the single elements to place my new one.
That worked, even if difficult and dangerous but the i had to collect the array again, using the pieces i've made using array_slice for the most.
Now i have everything in the right place, but something might have changed in the form, the result is that i can't see any info i gave when submitting the checkout form.
In the review screen i don't have addresses or any other information anymore, even if see them correctly inside the $form, the same obviously happens in the transactions list page.
Is there any other way to change the fields the checkout_form provides?

I paste here the code i used to put my array ($n3_fields) where i wanted it to be:

$address =array_slice($form, 0, 1);
$secondaparte =array_slice($form, 1, 300);
 foreach($address as $item) {
$shipping = array_slice($item, 0, 1);
$billing = array_slice($item, 1, 300);
 foreach($shipping as $fields ) {
$newaddress = array_merge(array_slice($fields, 0, 2,true),$n3_fields,array_slice($fields, 2, 300),$billing);
$recreate['address']['billing'][] = $newaddress;
}
}
$form = array_merge($recreate,$secondaparte);

I know it is quite twisted, but i had to recover the fieldset i lost dividing the array into fragments,
hope to find someone interested as much as i am in cusotmizing this $form
Thanks,
Da.