Denise,
I've been following this thread, and this morning I realized that two things may be getting mixed together.
I don't know your camera, but I will assume that you can magnify live view.
First, I think we can rule out a few things. The Tamron lens, assuming you have a good copy, is an excellent macro lens. No problem there. Mirror lock-up: may not be necessary, but certainly no harm. Shutter action: not an issue. Glasses--may make it hard to focus, but they won't make a blurry image look good. Maybe a problem for other reasons, but not the problem here, IMHO.
The first issue is this:
I and others took this to mean that the central area drops a bit after you position the camera. As several people have said, this is common. I used to use an inexpensive tripod head that seemed fine outdoors, but it drove me nuts doing macro work for precisely this reason. The problem wasn't that I wasn't locking it down enough; the problem was movement before the locking took effect.The central focus area always seems to move a fraction as I lock it.
There are several things you can do to deal with this. If you do enough macro indoors, you might find it worth buying a geared head, assuming your tripod can handle the weight. The amount of sag is trivial, but they are expensive (google Manfrotto junior 410), and if you want an arca clamp, you have to buy that extra. Also, given your arthritis, you'd want to make sure that you are comfortable rotating the knobs for the gears. If you don't do that, it may help to balance the weight more, as Grahame suggests. Also, you can aim slightly high before locking the head, and frame loosely enough that a small amount of movement won't move anything you want out of the frame.
However--this is what hit me this morning--while this affects framing, it should have very little effect on focus. The movement is along an arc, and in the limit--in very small amounts--it will approximate a simple vertical drop. Unless you are shooting at a very high level of magnification or with the aperture wide open, this shouldn't matter much.
So I am guessing now that the focusing issue is entirely separate. They key there may be the small size of the lcd, which will hide small errors of focus, and in macro work, small errors are all that is needed to make an image look blurry. I have been doing macro a long time, and even with a lot of practice, I often find that if I focus first with live view without magnification and then switch to 5X magnification, I discover that the focus that I though was right on the money is in fact not close enough.
So my suggestion is to start with something simple and see whether it works:
1. frame loosely enough that you have some room at the top, just in case.
2. aim slightly high before you lock down the head so that the sagging will move in the direction you want.
3. AFTER the motion has stopped, put the live view magnification area over the part of the image where you want sharpest focus, use at least 5 x magnification, and focus.
4. Avoid a small aperture.
See if that helps.
Dan