What you describe here was also a big part of my world view in the past. So I can understand you at least a bit. Let me answer your questions as good as I can.
1. Is it a good idea to just “admit” the reality of merit giving, despite any thoughts of it being contrived or “selfish”. Should I make a concerted effort every day to simply accumulate and dedicate merit however I can as a general gathi-cultivator??? Should I strive against any behavior that says otherwise?
One should learn the Buddha Dhamma and contemplate about it so that one can really feel joy related to giving. One needs compassion and a good heart. That comes with the understanding of the true nature of this world or Tilakkhana. You should do Satipattana and therefore act with sati all day. Whatever comes to the mind must be seen with sati, whether it is good/moral, neutral/functional, bad/unmoral. Discard all unmoral thinking, stop it with force if nothing helps. After a certain time your mind will know right from wrong.
Is giving mental/physical offerings on a personal altar any good at all? Do beings in paralowa/loka or upper realms really benefit from such things and what about a sotapanna abandoning religious activities? My wife claims to have (siddhi)powers of sight and while I act & believe in the spirit realms/deva realms I don’t know enough about it to live as if i’m interacting with it?
As long as one cannot perceive the other realms as real there is no need to force oneself to offer something. But you could train it, just a little offer every day, offered with compassion.
3.Is it a good idea to create a bhavana(?)/meditation that is something like “is what i’m doing right now something I want to happen again and again?” according to pati-icca samu-upada?
This is what we have to do. We cultivate our gathi in a way that there is no future bhava. This gathi is cultivated by understanding the true nature of this world. In this way one will lose all attachments. You do it with anapana and satipattana every day. You should do it in stting meditation as well as in daily activities. It need an effort or better it needs the satara iddhipada.
4.Is it a good idea to generate energy focusing on creating a gathi-character that “meditates often” despite the nuance of supramundane/mundane objects. and…
Is it a generally good idea to think of “creating a gathi” as a kind of useful tool in general, with the thought of ‘future’ jati and other births in mind? For instance with my worldly concerns it could be something like “I’m shifting my gathi to helping people with x-problem and by using -x solution as a gifts to my future births”???
Same as in #3 above. The habit of meditation brings a calm mind which can lose attachments easier. If you follow the 8fold path there is no need to cultivate another “rescuer-bhava”, instead you should cultivate Samadhi towards Nibbana, hence a bhava that drifts to total release. Maybe you should read (again) the Bhavana section on Lals website. I do not know which of the five hindrances (pancanivarana) is blocking you most. It only feels like thina middha and vicikicca. I really do not want to offend you in any way. Excuse me, if you feel uneasy because of anything I write.