Can we still reach loved ones with guidence after they have passed through the bardo into a rebirth?

@Monism Pantheist

I read your above added, 1) 49 days or 52 days? I would rather not to stuck with a concrete answer. But it's important to understand the significance of number. 49=7x7, 7 is an important number, we have 7 days a week, 7 colors in the rainbow... 7 is a number for a cycle. In the TTBoD you will find that every 7 days a more significant change. There is another sutra 《佛說入胎經》 which described how the embryo developed inside the womb, also in a 7 days cycle. And for female body, 7 years is a major cycle too, in general, eg, 7x2=14 a girl reached puberty, 7x7=49 a woman had menopause. And 3 is another number for a cycle. It is not easy to determine when exactly a person has left the body been in the Bardo stage. The doctor said heartbeat ceased means deceased, or breathe stopped, but some culture waited until the body is cold, the TTBoD said when the yellow fluid oozed from the body, etc. Thus 52 maybe is a safer indication since normal people do not have the ability to reach and communicate with the one already deceased to confirm when the cycle of leaving the body complete.

2) To "reach" a departed loved one after the Bardo: In Mahayana practice, many Buddhist Monasteries have a place to lit light (candle) for the departed, with his/her name on it. This could be for 1 day, 3 days, 7 days, or 49 days. The other way is to chant the mantra or sutra, and tranferred the merit to the departed. These are more ritualistic. Or to give away/ donate in the name of the departed, or his/her possessions, will add merit. However, in the buddhist teaching, everything is interconnected a thought with feeling is sort of energy, your thinking (blessing) directed to your grandmother will have effect on her, no matter whatever/ wherever she is.

Because the Buddhist view is that we have many deaths and rebirths in eons of kalpas therefore everyone could once be our father and mother, daughter and son, we should have compassion to all, and let go once a kinship ended. It is respectable you cared much about your grandma, however, after you have done all you know and can, that's the best of all.

You can send your blessings

But there are few conditions,

Your loved one must be in a life form which can receive your blessings.

Every person is not going to reborn as a human being . Being a human is very rare .

If your loved one reborn as a human or an animal he/she can not receive your blessings.

They can receive only if they reborn as a lower-level(lower than humans/animals) being .

In Buddhism if someone died , he will be born right after (maybe temporarily for few days and reborn in another life form) , in a way that a dying candle ignites another candle just after its death. The flame is the one which passes , not the candle , the candle can be different. Here , the candle is the life form (human,animal etc...), the flame is the soul that passes after the death.

Conclusion

You can contact your loved ones after death, but it has a one in a million chance .

But if you developed your mind to a certain point , you will be able to see what's going on after someone dies.

Note

Please don't read much books , because it has been a long time and most books have changed , deliberately or by mistake, it is very rare to find books with real Buddhism. So if you want to learn more I think it's best to learn from a teacher who has a practical knowledge(who has developed the mind to some level) about the path to nirvana.

"Can we still reach loved ones with guidence after they have passed through the bardo into a rebirth?"

If "guidance" you refer to what the book "Tibetan Book of The Death" instructed, then it would be "no". That person is in a new life form, all the possibilities during the bardo stage is conclusive now. In general saying, the bardo lasts max. 49 days. However, you may send blessings to the loved ones any time.