Probably. In some cases in HPLC you need at least 3 solvents (based on the solvent's polarity and the molecular target's solubility) to baseline separate molecules. If it works, RUN with it!

Sure - if it works for your separation you can use as many solvents as you like in your eluent system, as long as they mix. But personally I have never used more than 3. I would rather recommend testing solvent systems with addition of a little bit of acid or base or salt (depending on your sample). You could also try rerunning your TLC several times in different eluent systems, e.g. with increasing polarity. Or running a 2D TLC with different eluent systems.

You can do whatever you want as long as it gives you good reproducible and accurate results. Of course, it should be user-friendly and easy to do to minimize the operator error or environmental error.