Hello,
1 - ACL on price are used to provide a specific price for groups.
If you want to restrict the product to group ; best is to use the ACL on the product and not on its prices.
And by definition, a product without price is a free product. For sure you have a setting to forbidden the free products but it does not change the translation since a free product is still a product without a price.
Afterwards you might want to let your products visible for everybody and just allow specific groups to buy it ; then the solution you're using is the right one but yes, it requires some modifications in the translation to change the default behavior of an e-commerce system.
2 - The goal of the stock is to not allow customer to buy products out of stock.
HikaShop do not have "pre-sale" feature but it's in the TODO list.
You can find other thread in our forum talking about that feature request and some way to implement it manually.
For example : There are other ways to handle a display of a stock information without using the product quantity (which have the restrictions that you know) ; you can use a product custom field and display that field manually in the product page
Regards
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.