1. Never allow registration, always Guest as buyer
The Checkout Process starts on an Account Login page where visitors choose between Guest Checkout and Registering. We want to skip this step, and also eliminate all aspects of registration and membership on the site.
Method: redirect checkout links to Guest Checkout process, remove "Guest" titles from guest checkout process so that it looks like a standard checkout.
Redirect all Checkout buttons to point directly at the 1st step of "Guest Checkout".
Cart Page: Go to /template/pages/checkout/cart.tpl, change both instances of '<?php echo $checkout_rt; ?>' to 'checkout/guest_step_1'. Lines 71 & 183.
Cart-Top: Go to id_1/template/blocks/cart_top.tpl, change line 29 instance of href="<?php echo $checkout; ?>" to
href="/index.php?rt=checkout/guest_step_1". Note: If the cart_top modification breaks when site is uploaded, it's probably a root directory reference issue.
Remove the "Guest Checkout" titles from the two GC pages.
/pages/checkout/guest_step_1.tpl and also ...guest_step_2.tpl
Comment out line 2 <span class="maintext"><?php echo $heading_title; ?></span> in both files.
The checkout pages have a potentially un-needed sub-total. IF you don't charge shipping or taxes, there is no need to display a redundant sub-total.
You can't hack the sub-total out of the code because it is a required element in computing the total.
You also cannot turn-off Subtotal in the Order Totals extension because this results in a zero-balance Total for any order.
So the solution is to "not display" the sub-total.
The sub-total on the guest pages is called by a loop in the order_summary block.
You can remove the sub-total from the loop output with a single php line that says "only echo totals for which title not equal 'Sub-Total:' ".
Method:
Open /blocks/order_summary.tpl
Insert <?php if($total['title'] != 'Sub-Total:') { ?> at line 30
Also insert <?php } ?> at line 36.
2. Never shipping options
Remove Shipping Selection element from guest_step_2:
Insert a new style in style.css: ~~line 272 - #active_shippings {display: none;}
3. Never payment options
There is a default_cod payment extension included with AbanteCart.
I have never used COD or attempted your 3rd goal, but that is where I would start.
Good luck. Hope this helps.