I'm sure they thought of this and either had spare guns that were flint or they just stuck with flint if they thought it would be difficult to obtain. After all they had to think about powder availability also.
Sometimes a tiny bit of tin helps. A tiny bit will help fill out without making the lead harder. Also clean the mould and pin good and like others say, make sure the pin is hot. Some moulds with a separate pin and hard to get/keep the pins hot.
A 30-30 takes more deer every year than any other cartridge. If you do your part the 50 is plenty big for deer. If you think you may have to take a shot at over 100y I'd go with the 54 or if you think some day you may need to go for elk or other large critters.
If it did happen to get into the breech plug hole as you say, the cap would set it off and blow out the residue. By loading with a powder measure, and loose powder, you can adjust the grains more and likely get better accuracy.