Hi Jason, and welcome to Talk Cockatiels. 
I'm sorry you're having so much trouble with your tiels, unfortunately it takes a lot more time and patience to get somewhere with older birds. This doesn't make it impossible though, and that's the good thing.
It's important to realise when you buy a bird that not all birds want to be friendly with people, it's not in their nature. You need to accept this and be willing to look after them even if they don't want to be your cuddly pet. It comes with the job of being a slave to your birds. 
Something you might want to have a go at is something called clicker training. It's been used with a lot of success with all types of birds and all ages and with all temperaments. Clicker training rewards good behaviour and ignores the bad. You can buy a clicker cheap from most pet shops, and then you can make a start.
You need to be able to reward them, you say they won't take treats so maybe you can reward them by stepping away from them (assuming they're frightened by you) or just using verbal praise. Millet would be better, but if you can't do that you've just got to find something else that works. I would recommend the moving away option to begin with because it sounds like your tiels don't like you being too close. Have you ever tried feeding them sunflower seeds through the cage bars?
Anyway, once you have a clicker and a reward system you need to teach them to associate the sound of the clicker with the reward. This is called "charging" the clicker. To do this you click, and then reward. Do this several times until they seem to make the obvious connection that click = treat.
Once they have this idea in their mind you can start training.
The key to success with clicker training is to start slowly and move gradually. For example, if Pepper and Samantha thrash around madly when you stand by the cage, wait until they stop, click (mark the good behaviour) and then reward by moving away. Do this several times a day. You know they're responding when you can approach the cage without them flipping out.
Then you might start putting your hand in the cage, waiting for them to be calm, then click/reward. Eventually you'll move your hand towards them slightly, click/reward if they don't move away. Get the general idea?
It's UNBELIEVABLY important to move slowly, those examples of the different stages might take weeks to work through. If you do it right then your tiels will start wanting to hear the click and try to do what you want. In the end you can use that to get them to step up, or touch a target (like a wooden chop stick) wherever you put it (e.g. your arm or shoulder) by using clicker training. If you try to rush them through then they may get frightened and not want to try, so remember to work at their pace and not yours.
I've used this with my female tiel Cookie just for a bit of fun, and she cottoned on really quickly and would beak a target whether i held it up high, down low, left, right, etc. She really had fun with it. I think you could use this method with some success.
As to their diet, it can take weeks and even months to convert them to a healthy diet, and it's worth it in the end. I would ask some questions in the Food and Nutrition forum for tips on how to best do that.
Good luck.
I'm sorry you're having so much trouble with your tiels, unfortunately it takes a lot more time and patience to get somewhere with older birds. This doesn't make it impossible though, and that's the good thing.
Something you might want to have a go at is something called clicker training. It's been used with a lot of success with all types of birds and all ages and with all temperaments. Clicker training rewards good behaviour and ignores the bad. You can buy a clicker cheap from most pet shops, and then you can make a start.
You need to be able to reward them, you say they won't take treats so maybe you can reward them by stepping away from them (assuming they're frightened by you) or just using verbal praise. Millet would be better, but if you can't do that you've just got to find something else that works. I would recommend the moving away option to begin with because it sounds like your tiels don't like you being too close. Have you ever tried feeding them sunflower seeds through the cage bars?
Anyway, once you have a clicker and a reward system you need to teach them to associate the sound of the clicker with the reward. This is called "charging" the clicker. To do this you click, and then reward. Do this several times until they seem to make the obvious connection that click = treat.
The key to success with clicker training is to start slowly and move gradually. For example, if Pepper and Samantha thrash around madly when you stand by the cage, wait until they stop, click (mark the good behaviour) and then reward by moving away. Do this several times a day. You know they're responding when you can approach the cage without them flipping out.
It's UNBELIEVABLY important to move slowly, those examples of the different stages might take weeks to work through. If you do it right then your tiels will start wanting to hear the click and try to do what you want. In the end you can use that to get them to step up, or touch a target (like a wooden chop stick) wherever you put it (e.g. your arm or shoulder) by using clicker training. If you try to rush them through then they may get frightened and not want to try, so remember to work at their pace and not yours.
I've used this with my female tiel Cookie just for a bit of fun, and she cottoned on really quickly and would beak a target whether i held it up high, down low, left, right, etc. She really had fun with it. I think you could use this method with some success.
As to their diet, it can take weeks and even months to convert them to a healthy diet, and it's worth it in the end. I would ask some questions in the Food and Nutrition forum for tips on how to best do that.
Good luck.