Yes. You can seat bullets past the datum line.
The amount of jump to the lands is often a factor of accuracy. Varying the amount of jump is part of load development and can also affect pressures. Some rifle/bullet combos prefer a lot of jump and some prefer to be slightly jammed into the lands. Most like just a little .010". Either way, you need to start with minimum charges and work up to it.
The degree to which the powder fills the case may also be a factor in accuracy. With too much empty space, the powder isn't always lying in the same location and doesn't necessarily ignite/burn as consistently as it might with a full case. Hence, different powders afford the option to better utilize case capacity while matching the most efficient burn rate, harmonics, etc.
Compressed loads aren't inherently bad. But, they need to be approached and managed carefully and worked up to.
When developing loads, you always need to factor in some safety margin. Increased ambient temperature will increase pressures. So, maximum loads developed in cold weather may create pressure problems when fired in hot weather.
The "datum line" that I think you are referring to is for measuring headspace between that point on the shoulder to the bolt face. It really doesn't have anything to do with seating depth.
You need enough bearing surface between the neck and bullet to hold it in place and to keep the bullet from falling out or wiggling. Seating the bullet any deeper is of no real benefit and only uses case capacity.
Hope this helps.
Richard
