Hey,
Sorry for the delay.
Regarding the first issue.If you are using my projectiles and damage calculators, there is a list of player projectile actors with blueprints(bp__player_projectile and bp_player_meeleattackdamage and all of them have a destroyactor node. Just change it to a set life span node and add whatever delay you want).
For the second issue, see Player_character blueprint, there is a section where the player changes color. Just replacethe damage flash color variable color with whatevber you want( any color with an alpha value near 0 should work)