Regarding the 'backwards splitter' to be used as a combiner to add a modulator channel to OTA, I have had much better performance using a high frequency splitter as a combiner (2400 MHz) instead of a regular splitter (900 or 1000 MHz). There is less loss and/or a more efficient coupling of the 2 inbound signals to the output.
Also, if all your OTAs are UHF, and your modulator is a channel 3 or 4 unit, a simple (although harder to find) VHF/UHF band splitter works great too.
As for combining your signals before or after any RF amp you have, you'll just have to try both and see what works best. One nice thing about quite a few of the new ATSC HD sets is a pretty good signal strength meter in the TV menu somewhere. it can be a very handy feature when doing work on your OTA system. Just leave meter on, and modify your set up one step at a time and review your results as you go. You make a mistake, and you'll know immediately and can fix it. If you change 10 things all at once and nothing works you might be troubleshooting a while before you find that one coax connector with the little whisker in it, or the short center conductor that is killing your set up.
The Sammy's just have 8 or 10 bars (don't recall off hand) but some Sonys have real nice ones with 0-100 scale, and even S/N ratio or capture (again don't recall precise menu terminology).