Module development and code questions

I know there is big discussion about meta tags... but is there way to add meta tags and description to 4.7?

In the context of trying to find documentation for the 'prepare' phase of node form editing, I'm stuck.

I'm trying to upgrade the HTML Tidy module (Which appears to be abandoned, so I'm gonna comandeer it)

I've done a 4.6 version which validates and corrects users input BEFORE it gets saved in the database, not after (like most of the filters do).
I just don't want invalid HTML to make it into the system.

I am having problems understanding something.. i fear it might be important.

when would i use nodeapi $op = fields.. and what would it give me?

i have read things like :

'fields' is used to
add extra db-fields to be saved together with the node, before its
really stored.

but what does that mean exactly?

Has anyone integrated mailman or some other mailing list application?
I need to find a way to create a block where users can sign up for a mailing list, and once they have signed up, have that block not appear.
Possible?
Done before?

The generic module (a sub-module of E-Commerce) simply uses:

case 'is_shippable':
   return false;

to get rid of things to do with shipping. However, it seems the equivalent code does not exist for hiding prices.

(1) How is it possible to 'hide' the prices of products? That is, the price input field should not be mandatory, and there should be an option for hiding/showing the price.

(2) How is it possible to skip the payment step in the checkout process?

Okay, so this will be the home of the project to bring multi-level paid subscriptions with automatic Paypal recurring debits.

Here are our project requirements.

(This is just a kickstart to the discussion)

(From a discussion with ae2005)

1. 4.6<\li>
2. automatic recurring debits via Paypal's recurring subscriptions function<\li>
3. ability to also use ecommerce module simultaneously (After debating the pros and cons, I personally think it'd be best to make it a contrib module to ecommerce, because I expect that they will support that module through all future releases) There are people on the thread who want it to not require ecommerce. what are your thoughts?<\li>
4. ability to still sell subscriptions via a one-time item paypal button (because that button will accept credit cards from people without a paypal account, whereas paypal's recurring subscription feature requries a paypal account)<\li>
5. more than one type of subscription (unlimited would be great.)<\li>
6. automatic removal upon expiration via cron<\li>
7. automatic upgrade to correct role before cron is run (ecommerce does this now, but not for multiple roles)<\li>
8. abililty to add people manually (e.g. people who mail in a check.) I believe ecommerce can do this currently via the cod module, but I haven't tested it.<\li>
9. correct timezones. (right now for paypal subscription to work I have to set timezones to gmt and disable local timezones.)<\li>