When and why did the ancient church no longer care about offending a brother?

Your question makes a lot of assumptions, none of which are necessarily true.

Your first assumption is a fairly common one, that the early church was perfect in every way and did everything that was preached, living up to its ideals all the time. This was probably not the case - Paul wouldn't have had to write about this twice if the church had always been doing it. In other words: this may not have been something that Christians always did in New testament times, and then somehow stopped doing; it may have been something that Christians always knew they were supposed to do, but have found really difficult in practice - both now and in New Testament times.

Secondly you write that "there must have been some Jewish Christians in the fourth Century". Actually there probably weren't. The split between Christianity and Judaism was pretty definite long before that. There probably were Christian of Jewish descent, but they would have been thrown out of their synagogues and ostracized. Remember that Christianity teaches that it is no longer necessary to adhere to Jewish rituals and customs, even for Jews, once you become a Christian. I very much doubt that there were many Christians who sincerely thought that it was necessary to adhere to Jewish ethics (as opposed to Christian ones) by 400AD.

Your point about 'halos' has already been answered in this question. They are just an artistic convention, and there is no evidence that using them ever 'gave offence' to anyone.

Christian representational art has been found dating from the second century AD, so we can presume it was common at that point. I know of nothing to indicate that this art was an "aggressive introduction" as you put it, or that there was objection to it from Christians of Jewish descent.

Jesus came on the scene and brought full understanding. Before he came full understanding was veiled (hidden behind the veil - not understood and not revealed fully).

Jesus said, "I didn't come to do away with the law but fulfill it" (Matthew 5:17). Deeper revelation and understanding came with Christ, and with his death and resurrection.

The holy spirit came to teach us or lead us into all truth. Paul realized the truth of the matter. True understanding of the teaching of not defiling your self its not about what you eat or drink, or how you wash your hands, but about your inner man and attitude.

If you remember the Old Testament laws about eating and drink mainly were about defiling self, so I'd say it's not so much that the Church stopped doing it, but instead, they gained better understanding of the Biblical truth set before them in the old testament laws

The fact that Paul found it necessary to discuss this issue not once, but twice, would seem to indicate that it WAS a problem in the first century church, and not something that the church handled just fine in the first century but then went astray later.

More important, there are two sides to Paul's discussion of such offense. He doesn't say, If something offends a brother, don't do it, period end of story. Rather, he has admonitions for both parties.

Romans 14 2 For one believes he may eat all things, but he who is weak eats only vegetables. 3 Let not him who eats despise him who does not eat, and let not him who does not eat judge him who eats; for God has received him. 4 Who are you to judge another’s servant? To his own master he stands or falls. Indeed, he will be made to stand, for God is able to make him stand. ... 19 Therefore let us pursue the things which make for peace and the things by which one may edify another. 20 Do not destroy the work of God for the sake of food. All things indeed are pure, but it is evil for the man who eats with offense. 21 It is good neither to eat meat nor drink wine nor do anything by which your brother stumbles or is offended or is made weak.

So yes, vv 19-21 say that the brother who sees nothing wrong with X should nevertheless be careful that he does not create a stumbling block for the brother to whom it is a problem. This may mean giving up something that he is convinced is perfectly good and acceptable to avoid offending another Christian.

But before he says that, look at verse 3: The brother who thinks that X is bad but has no clear scriptural justification for saying that, should not condemn the brother who does it.

We could debate exactly what Paul meant you should do. I suspect he's a little vague because the answer is not so simple as "never do it". If that was the rule, then one foolish Christian who condemns things for no good reason would force all the other Christians in the world to give up all sorts of good things. It is no answer to say that you will only give it up if the other person can offer good reasons. Paul's whole point is that he is talking about "disputable matters". By definition, this is about cases where neither person can give solid, undisputable reasons for his position.

Suppose another Christian said that he believed that a Christian should not own a house. Would you burn your house down and live under a bridge to avoid offending him? (You can't sell it -- that would just be encouraging others to sin!) Sorry, I doubt I would go that far.

In practice, well, let me take a real case. Many Christians I know believe you should not drink any alcohol at all. I disagree. I won't get into the scriptural arguments here as they're beside the point. I don't belittle Christians who don't drink. I don't try to badger them into drinking, like saying, "Here, have some wine. How can it hurt? What's the big deal?" etc. I don't drink in front of them, offer them alcohol, or discuss how much I like this brand or whatever in front of them. But I don't totally abstain to avoid offending hem, either. I just don't do it in front of them.

Likewise, Paul is saying that they should not condemn me for drinking.

We should respect each other's difference of opinion or understanding.