Absolutely true, God can be relied upon.
With all due respect, what I see you doing is forcing pieces to fit. You take the pieces above and they tell you God cannot cut off a branch He added. So you have to say the branch added itself which is impossible and not supported by scripture. God alone does the adding and taking away.
May I suggest taking a closer look at what Paul is telling us.
17 But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree,
We see God does the breaking off and the adding. The wild branches are being nourished. They are grafted in and being supplied.
20 That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear.
We see they stand fast through faith. If their faith wasn't good enough they wouldn't have been added and they wouldn't be standing fast.
22 Note then the kindness and the severity of God: severity toward those who have fallen, but God's kindness to you, provided you continue in his kindness. Otherwise you too will be cut off.
Paul says provided you continue otherwise. It's up to them to continue.
2 Peter 1 Tells us to add to our faith so that we may not fall.
5 For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, 6 and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, 7 and godliness with brotherly affection, and brotherly affection with love. 8 For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. 9 For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. 10 Therefore, brothers, be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall.
The Bible says that a branch can add itself - temporarily. That is why they fall away because it was not by God that they were added. Otherwise, the verses that I posted simply couldn't be true - it cannot be both ways: that a branch is added by God and yet it falls away.
Branches that temporarily add themselves, Christ never abided in them, even though the for a time they chose to claim Christ. This is the difference: that many claim Christ but few are claimed by Christ. Only those claimed by Christ can truly claim Him- they alone are the true branches, and by which, they alone become those who abide and bear much fruit: unless Christ first abide in someone, they cannot remain in Him; that is, they can only abide in Him because He first abided in them, and if He did, then they cannot be removed as a branch; if He didn't, then they must be removed. They only "abide in Him", IF His words also abide in them - only those in whom He abides, will His words first abide in them - that is how His abiding is manifested. Therefore, those who did/do not become His disciples were never the branches and are cast away. The key is that Christ must first abide in them. From/by that, everything else follows.
May I respectfully suggest you read these verses closely.
[Jhn 15:4-8 KJV]
4 Abide in me,
and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. 5 I am the vine, ye [are] the branches: He that abideth in me,
and I in him, the same bringeth forth much fruit:
for without me ye can do nothing.
6 If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast [them] into the fire, and they are burned.
7 If ye abide in me,
and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
8 Herein is my Father glorified, that ye bear much fruit;
so shall ye be my disciples.