I added -25 to base reluctance but still it needs to use other factors like in EU4 like money, profits etc. In EU4 you may grant a county to the allies but here mechanics are cut.

Looks like -50 to base reluctance is just fine: you get 0 to most acceptances when your relationships are 100 and AI has no real interest in helping you.

Also need to add something like
if = {
limit = {
scope:hook = yes
has_hook = scope:recipient
}
use_hook = scope:recipient
}

to the

auto_accept = {

in r_defensive_alliance.txt

to fix the hook usage