Hi Mary,
For a tincture internally, absolutely plantain would make a good substitution for comfrey.
While I haven’t heard specifically of shepherd’s purse externally for hiatus hernia, I can imagine how it would be of support. I would also suggest gotu kola (both internally and/or externally), marshmallow root internally and you could do some comfrey externally. Supporting digestion overall (chamomile, marshmallow root, meadowsweet-caution around surgery…) will be your goal. This also includes liver supportive herbs (dandelion, milk thistle…). Often the person will need surgery, but any and all of these herbs can be of such good help in supporting the person through the process.
For tincture formulas making them separately and then mixing the tinctures together at the dose recommended for each herb is the best practice. This allows for each plant to have as full an extraction as possible and you have a better chance of knowing your dose within the formula.
With warmth,
Benna