Replying to your points. . .
1. Wow, I'd be a bit concerned about that dose myself. As previously noted, there is no safety data on doses over 120mg and it's not typically recommended. Are you working with an IBCLC? It would be a good idea to be discussing dosage and options with a breastfeeding professional, especially with increasing dose beyond typically recommended levels.
2. Every 3 hours sounds very normal. Most breastfed babies are going to nurse frequently when they have the option. I would think moving to EP'ing would be more likely to lead to an even lower supply. Unless your LO is not removing milk well at the breast, a nursing baby is more effective than any pump. Getting 0-1oz when pumping after nursing is a good sign that your DS is doing a good job removing milk effectively. Can you keep up your pumping routine on at least some of the days when you don't work? That can help boost supply and keep your body used to the pumping routine. What about nursing instead of pumping for the bedtime feed? Again, he's going to be more effective at removing milk than the pump.
3. Good on checking the pumps just in case. It's unlikely that both would be failing at the same time, though, unless you're sharing parts between them. A pump retailer (usually an IBCLC who also sells pumps) can test the suction on them to ensure the pump motors are working well.
5. Yep, slow is better w/ solids if food allergies are a concern. And until he's around a year (and even longer for many nursing toddlers, especially those w/ food allergies), your milk should make up the majority of his diet. But if you are offering solids, then those will displace some amount of milk in his diet. Having your caregiver predominately feed any solids you're including in his diet will offset the amount of milk he'll need while you're at work.
You mention slow weight gain. Is his gain being measured using the WHO growth charts? The CDC charts which are still being used by most US docs are based on predominately formula-fed babies, and the growth patterns in those charts are NOT normal, especially after the first 3-4 months when breastfed babies growth is supposed to slow down. The CDC actually recommends against using their charts, and recommends in favor of the WHO charts as representing what we now consider best feeding practices and growth expectations.