What are the risks of elevating the legs too regularly and when tired?
To answer your questions, one needs to address certain issues one at a time.

Veins do have valves, but they function to hold blood from settling in the legs. When you flex muscles, the blood flows towards the heart. Valve failure occurs when people stand (especially if they stand relatively still) for long periods of time every day over many years, dilating veins. You can see this on the surface of the legs as varicose veins. One way to treat varicose veins is with elevation. So, elevation does not harm vein valves.

Lymph is made at the arterial end of capillaries and is part of the interstitial fluid of tissues. It bathes cells, and returns to the circulation via the lymphatic system (which also has one-way valves), goes through lymph nodes, and returns to the blood via connections with the subclavian veins. The treatment for too much lymph in the lower extremities (also known as edema) is to elevate the legs. So, elevation does not harm the lymphatics system.

The heart was made to pump all day, every day, for many, many years. It doesn't get "tired" during a tiring day, and doesn't need to rest like we do. But it can get weak and pump inefficiently if injured (e.g. myocarditis, myocardial infarct), or if it pumps against high arterial pressure (hypertension).

As long as you have healthy arteries to the legs, I can't see how your legs can suffer by elevation.

The main effect that I can think of is on baroreceptors. The increased volume of blood in your upper body will cause an increased stroke volume from the heart, sensed by baroreceptors, and may cause smooth muscle dilation of the vascular system. In the short term, that means you will experience more pronounced orthostatic hypotension on standing (you will be dizzier). The long-term effect from elevation for a couple of hours a day are basically nil.

Since you asked about these parameters only, I won't elucidate on other possible effects.