How to fix a chipped acrylic tub

I believe you just set it on the floor and let the weight of the tub and connected drain pipes keep it from moving.

If you are really worried about it moving you could mount some sort of small block to the floor behind where each leg will sit. It would not be attached to the blocks but with for of them the tub would not be able to move. But that may not work well with your tile floor.

Clearly the restriction is left of the sink in your diagram. One more question you could consider is whether this is an installation deficiency (problem has always existed) or whether the drain was working properly at one time and the situation has become worse.

I would suggest using a video inspection camera to be certain about the location of the restriction and perhaps what is causing the restriction. Here is a rental unit that may be available in your area. Or here is a contractor that could provide this service. It would be best to try to clear debris before the blockage prior to a camera inspection.