1. all filters have a by-pass valve that is supposed to open if/when the filter gets clogged. this is so that your filter will not explode under pressure. by placing it on the suction side, your pump is going to create enough suction to open the by-pass valve, thereby rendering the filter useless. and, even IF it did work, once it got clogged and IF the by-pass failed, you'd collapse your filter. i don't care how big the suction line is or how big the filter is.
2. we've had numerous discussions here about that already. there are only special circumstances where a filter is on the suction line and on a splitter is NOT one of them. usually it would be on high tech equipment for various reasons.
3. a properly designed tank would always have the suction port off the bottom of the tank for that reason: so as to not suck any particles that might be in there. some even put in-line screens on the suction line, but most tanks don't offer access to change them if you ever had to, not to mention if they got clogged, it would ruin your pump from lack of oil flow.
4. bottom line is to take care never to allow anything to get inside your tank.