The only way I have used keiki paste is to kick-start dormant eyes in Catts and Oncids, and it definitely works, both eyes on older back-bulbs and getting extra eyes to pop on lead pseudobulbs. The plant needs to be healthy enough to do what you are pushing, so it won't necessarily revive a nearly dead back-bulb. And it isn't likely to make eyes on active pseudobulbs start out of season, but if you time it right just ahead of the normal start of growth you can usually get 2-3 eyes going at once instead of 1, or eyes on the second and third bulb back to start too - if the plant is healthy and vigorous in the first place.

For keiki paste on Phals I can only speak from theoretical knowledge and received wisdom, but from 40+ years growing orchids and listening to other experienced growers, a botany related degree and experience working with plant hormones in a plant tissue culture lab, here's the theory. If a plant has been triggered to be in flowering mode, as in a fresh spike or one just finishing flowering, you will probably trigger more flowers. If a spike is still green a few weeks or longer after the last flowers are gone and shows no signs of branch spikes then you have a good chance of making keikis. I would use the lowest nodes on the spike, and put the keiki paste on several nodes. Not all may start so you want as many chances as possible, and it is not true that producing keikis stresses a healthy plant. As soon as they have a leaf they are contributing to overall plant strength so as long as the roots are healthy, fertilizer is adequate and the plant gets general good care many keikis just means a healthy actively growing plant. Growth is good. It may slow down growth of the main plant or delay the next flowering cycle. That doesn't mean the plant is stressed, just that it has redirected resources the way you asked it to.